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A  DESCRIPTIVE  CATALOGUE  of  all  these  and  many  more  may  be  obtained  by  enclosing  a 
stamp  to  the  Publishers, 

A.  S.  BARNES  &  COHPAKY, 

National  Educational  Publishers, 
111   &  113  WILLIAM  STREET,  NEW  YORK. 


THE  WOBMAH  SERIESJNJODEPJ  LANGUAGE,    . 

A  COMPLETE  COURSE  IN  GERMAN. 

By  JAMES  H.  WORMAN,  A.M.  -;V 


EMBKACINa 


GRAMMAR, 


COPY-IBOOKS,  GERMAN"    ECHO. 

IN  PREPARATION, 

HISTORY    OB1    GERM^lNr    LITERATURE, 

GERMAN    J^NID    ENGLISH    LEXICON. 

J.  TH"2£  GERMAN  GRAMMARS  of  Worman  are  widely  preferred  on  ac 
count  of  their  clear,  explicit  method  (on  the  conversation  plan),  introducing  a  system 
of  analogy  and  comparison  with  the  learners'  own  language  and  others  commonly 
studied. 

Th  -3  arts  of  speaking,  of  understanding  the  spoken  language,  and  of  correct  pronun 
ciation,  are  treated  with  great  success. 

The  new  classifications  of  nouns  and  of  irregular  verbs  are  of  great  value  to  the 
pupil.  The  use  of  heavy  type  to  indicate  etymological  changes,  is  new.  The  Vocabu 
lary  is  synonymical  —  also  a  new  feature. 

II.  WORMAN'S    GERMAN  READER    contains    progressive    selections 
from  a  wide  range  of  the  very  best  German  authors,  including  three  complete  plays, 
which  are  usually  purchased  in  separate  form  for  advanced  students  who  have  com 
pleted  the  ordinary  Header. 

It  has  Biographies  of  eminent  authors,  Notes  after  the  text,  References  to  all  Ger 
man  Grammars  in  common  use,  and  an  adequate  Vocabulary  ;  also,  Exercises  for 
translation  into  the  German. 

III.  WORMAN'S   GERMAN  ECHO  (Deut$ches  Echo)    is    entirely  a  new 
thing  in  this  country.     It  presents  familiar  colloquial  exercises  without  translation, 
and  will  teach  fluent  conversation  in  a  few  months  of  diligent  study. 

No  other  method  will  ever  make  the  student  at  home  in  a  foreign  language.  By  this 
he  thinks  in,  as  well  as  speaks  it.  For  the  time  being  he  is  a  Gennan  through  and 
through.  The  laborious  process  of  translating  his  thoughts  no  longer  impedes  free 
unembarrassed  utterance. 


WORMAN'S  COMPLETE  FRENCH  COURSE 

IS  INAUGURATED  BT 

Ij'ECHO      IDE      DP  .A.  IR,  I  S ., 

Or,  "French  Echo;'1  on  a  plan  identical  with  the  German  Echo  described  above. 
This  will  be  followed  in  due  course  by  the  other  volumes  of 

THE    FRENCH   SERIES, 

viz.: 

A   CO  MPT,  ETE  GRA  MMAR,  \A    ERE  NCn    REA.DER, 

AN  ELEMENTARY  GRAMMAR,  \  A    FRENCH    LEXICON, 
A.   HISTORY  OF  FRENCH  LITERATURE. 


WORMAN'S    WORKS 

are  adopted  as  fast  as  published  by  many  of  the  best  institutions  of  the  country.    In 
completeness,  adaptation,  and  homogeneity  for  consistent  courses  of  instruction,  they 

are  simply 


•M 


<J-rf 


POPULAR    PHYSICS, 


PECK'S    GANOT. 


» 
INTRODUCTORY  COURSE 


OF 


NATURAL  PHILOSOPHY 


FOR   THE    USE   OP 


SCHOOLS    AND    ACADEMIES. 


EDITED   FROM 


GANOT'S  POPULAR  PHYSICS, 

BY 

WILLIAM  G.  PECK,  LL.D., 

PROFESSOR  OF  MATHEMATICS    AND    ASTRONOMT,  COLUMBIA    COLLEGE,  AND  OF 
MECHANICS  IN  THE    SCHOOL  OF  MINES. 


A.  S.  BARNES  &  CO., 
NEW   YORK   AND    CHICAGO. 

1871. 


KDCCATIOI  LIBR. 
THE    NATIONAL    SERIES 


OP 


STANDARD  TEXTS  IN  THE  SCIENCES. 


I.— NORTON'S  FIRST  BOOK  IN  PHILOSOPHY. .  .$1  00 
PECK'S  GANOT'S  NATURAL  PHILOSOPHY. .  1  75 

H.— PORTER'S  FIRST  BOOK  IN  CHEMISTRY 1  00 

PORTER'S  PRINCIPLES   OF  CHEMISTRY....  2  00 

HI.— J  ARVIS'  PRIMARY  PHYSIOLOGY 75 

JARVIS'  PHYSIOL.  AND  LAWS  OF  HEALTH. .  1  75 

IV.— WOOD'S  OBJECT  LESSONS  IN  BOTANY 1  40 

WOOD'S  CLASS-BOOK  OF  BOTANY 3  50 

V.— STEELE'S  14  WEEKS  IN  ASTRONOMY 1  50 

VI.— PAGE'S  ELEMENTS  OF  GEOLOGY 1  25 

VIL— CHAMBERS'  ELEMENTS  OF  ZOOLOGY 1  50 


THESE  STANDARD  WORKS 

AEE  FOE  SALE  BY  ALL  BOOKSELLEKS, 

Or  may  be  procured  from  the  Publish ers  by  Mail,  postpaid, 
on  receipt  of  price. 

.A..  S.  JBAR1VES  «fc   CO., 

NEW  YORK. 


Entered  according  to  Act  of  Congress,  in  the  year  1860, 

BY  WILLIAM  G.  PECK, 

In  the  Clerk's  Office  of  the  United  States  District  Court  for  the  Southern  District  of 
New  York. 


GIFT 


QCZ 


Lit 
PREFACE. 


THE  rapid  spread  of  scientific  knowledge,  and  the  con 
tinually  widening  field  of  its  application  to  the  useful 
arts,  have  created  an  increased  demand  for  new  and 
improved  text-books  on  the  various  branches  of  NATU 
RAL  PHILOSOPHY. 

Of  the  elementary  works  that  have  appeared  within 
a  few  years,  those  of  M.  GANOT  stand  preeminent,  not 
only  as  popular  treatises,  but  as  thoroughly  scientific 
expositions  of  the  principles  of  PHYSICS.  His  "Traite 
de  Physique"  has  not  only  met  with  unprecedented 
success  in  France,  but  has  been  extensively  used  in 
the  preparation  of  the  best  works  on  Physics  that  have 
been  issued  from  the  American  press. 

In  addition  to  the  "Traite  de  Physique,"  which  is 
intended  for  the  use  of  Colleges  and  higher  institutions 
..of  learning,  M.  GANOT  has  recently  published  a  more 
^elementary,  work,  adapted  to  the  use  of  schools  and 
academies,  in  which  he  has  faithfully  preserved  the 
prominent  features  and  all  the  scientific  accuracy  of  the 
larger  work.  It  is  characterized  by  a  well-balanced 

925 


Vl  PREFACE. 

distribution  of  subjects,  a  logical  development  of  scien 
tific  principles,  and  a  remarkable  clearness  of  definition 
and  explanation.  In  addition,  it  is  profusely  illustrated 
with  beautifully  executed  engravings,  admirably  calcu 
lated  to  convey  to  the  mind  of  the  student  a  clear 
conception  of  the  principles  unfolded.  Their  complete 
ness  and  accuracy  are  such  as  to  enable  the  teacher 
to  dispense  with  much  of  the  apparatus  usually  em 
ployed  in  teaching  the  elements  of  Physical  Science. 

In  preparing  an  American  edition  of  this  work  on 
POPULAR  PHYSICS,  it  has  not  been  the  aim  of  the 
editor  to  produce  a  strict  translation.  No  effort,  how 
ever,  has  been  spared  to  preserve  throughout,  the  spirit 
and  method  of  the  original  work.  No  changes  have 
been  made,  except  such  as  have  seemed  calculated  to 
harmonize  it  with  the  system  of  instruction  pursued  in 
the  schools  of  our  country. 

By  a  special  arrangement  with  M.  GANOT,  the  Amer 
ican  publishers  are  enabled  to  present  fac-  simile  copies 
of  all  the  original  engravings. 

NEW  YORK,  June  1st,  1860. 


NOTE. 

At  the  request  of  many  teachers,  a  chapter  has  been  pre 
pared  on  the  Application  of  Physical  Principles  to  Machines. 
In  the  revised  edition  several  cuts,  with  their  corresponding 
numbers,  have  been  omitted. 

NEW  YORK,  June  1st,  1871. 


CONTENTS. 


INTRODUCTION. 

CLASSIFICATION  OF  THE  SCIENCES. 

CHAPTER   I. 

PRELIMINARY   PRINCIPLES  AND  MECHANICS  OF  SOLIDS. 

I  — DEFINITIONS  AND  GENERAL  PROPERTIES  OF  MATTER 11 

II. — MECHANICAL  PRINCIPLES 22 

HI. — PRINCIPLES  DEPENDENT  ON  THE  ATTRACTION  OF  GRAVITA 
TION 39 

IV. — PRINCIPLES  DEPENDENT  ON  MOLECULAR  ACTION 64 

V. — PROPERTIES  OF  SOLIDS  DEPENDENT  ON  MOLECULAR  ACTION  70 

CHAPTER   II. 

MECHANICS   OF  LIQUIDS. 

I. — GENERAL  PRINCIPLES 73 

II. — EQUILIBRIUM  OF  LIQUIDS 82 

III. — APPLICATIONS  OF  THE  PRINCIPLE  OF  EQUILIBRIUM 86 

IV. — PRESSURE  ON  SUBMERGED  BODIES 90 

V. — SPECIFIC  GRAVITY  OF  BODIES 96 

CHAPTER   IN. 

MECHANICS   OF  GASES   AND   VAPORS. 

I. — THE  ATMOSPHERE 105 

IT. — MEASURE  OF  THE  ELASTIC  FORCE   OF  GASES 124 

III. — APPLICATION  TO  PUMPS  AND  OTHER  MACHINES 128 

IV. — APPLICATION  TO  BALLOONING 149 

CHAPTER   IV. 

ACOUSTICS. 

I. — PRODUCTION  AND  PROPAGATION  OF  SOUND 156 

II. — MUSICAL  SOUNDS 1 69 

CHAPTER   V. 

HEAT. 

I. — GENERAL  PROPERTIES  OF  HEAT 188 

II.  —THERMOMETERS 187 

III. — RADIATION  OF  HEAT 196 

IV. — REFLECTION,  ABSORPTION,  EMISSION,  AND  CONDUOTIBILITY  198 

V. — LAWS  OF  EXPANSION  OF  SOLIDS,  LIQUIDS,  AND  GASES  . .  210 

VI. — CHANGE  OF  STATE  OF  BODIES  BY  THE  ACTION  OF  HEAT  219 

VII. — VAPORIZATION. — ELASTIC  FORCE  OF  VAPORS 223 


Vlll  CONTENTS. 

PAGH 

VIII    -CONDENSATION  OF  GASES  AND  VAPORS.-  -SPECIFIC  HEAT  238 

IX   — HYGROMETRY. — RAIN. — DEW. — WINDS 245 

X  —SOURCES  OF  HEAT  AND  COLD 255 

CHAPTER  VI. 

OPTICS. 

I.-  GENERAL  PRINCIPLES 259 

II.-  REFLECTION  OF  LIGHT. — MIRRORS 267 

III.-  REFRACTION  OF  LIGHT. — LENSES 283 

IV  -  -DECOMPOSITION  OF  LIGHT. — COLORS  OF  BODIES 311 

V  --THEORY  AND  CONSTRUCTION  OF  OPTICAL  INSTRUMENTS..  323 

CHAPTER  VII. 

MAGNETISM. 

I.   -GENERAL  PROPERTIES  OF  MAGNETS 358 

JL-  —TERRESTRIAL  MAGNETISM.  —COMPASSES 366 

(II.  -METHODS  OF  IMPARTING  MAGNETISM 371 

CHAPTER  VIII. 

STATICAL  ELECTRICITY. 

I  -  FUNDAMENTAL  PRINCIPLES 376 

II.  —PRINCIPLE  OF  INDUCTION. — ELECTRICAL  MACHINES 386 

III.  —ELECTRICAL  RECREATIONS 397 

IV.  -ACCUMULATION  OF  ELECTRICITY 406 

V.-  -EFFECTS  OF  ACCUMULATED  ELECTRICITY. 415 

VI.  -ATMOSPHERIC  ELECTRICITY ...  421 

CHAPTER  IX. 

DYNAMICAL  ELECTRICITY. 

I. — FUNDAMENTAL   PRINCIPLES 432 

II. — APPLICATIONS  OF  GALVANIC  ELECTRICITY •  438 

CHAPTER  X. 

ELECTKO-MAGNETISM. 

I. — FUNDAMENTAL  PRINCIPLES 451 

II. — ELECTRO-MAGNETIC   TELEGRAPHS. — THE  ELEOTRO-MOTOB  459 

HI  — INDUCTION. — APPLICATION  TO  MEDICINE 469 

CHAPTER  XI. 

APPLICATIONS  TO  MACHINES. 

I. — GENERAL  PRINCIPLES 475 

II. — ELEMENTARY  MACHINES 477 

III. — HURTFUL  RESISTANCES 483 

[V. — WHEELWORZ  . . . , 485 

V. — REGULATORS 487 

VI.  —PRIME  MOVERS  , 488 


POPULAR   PHYSICS. 


INTKODUCTIOlSr. 

CLASSIFICATION     OF     THE     SCIENCES. 

SCIENCE  is  a  knowledge  of  the  laws  that  govern  the 
Universe. 

A  Law  is  a  necessary  relation  between  cause  and  effect. 
It  is  assumed  as  the  foundation  of  all  Science,  that  Wee  causes 
produce  like  effects.  This  principle  is  an  inductive  truth, 
founded  upon  universal  experience. 

By  the  UNIVERSE  we  mean  all  that  has  been  created, 
whether  material  or  immaterial.  The  Universe  may  be 
regarded  as  made  up  of  mind  and  matter.  MIND  is  that 
which  thinks  and  wills ;  MATTER  is  that  of  which  we  become 
cognizant  through  the  medium  of  the  senses.  Science  ad 
mits  of  two  corresponding  divisions,  Science  of  Mind,  or 
METAPHYSICS,  and  Science  of  Matter,  or  NATURAL,  PHI 
LOSOPHY. 

NATURAL  PHILOSOPHY  is  that  branch  of  science  which 
treats  of  the  laws  that  govern  the  material  Universe. 

Matter  exists  in  two  states,  organized  and  unorganized ; 
it  is  organized  when  its  particles  are  aggregated  into  organs 
adapted  to  the  support  of  life  ;  in  all  other  cases  it  is  un- 

What  is  Science  ?  What  is  a  Law  ?  Define  the  Universe.  Mind.  Matter.  What 
are  the  two  divisions  of  Science  ?  What  is  Natural  Philosophy  ?  In  what  two  states 
may  Matter  exist  ?  Illustrate. 


10  POPULAR   PHYSICS. 

organized.  Natural  Philosophy  admits  of  two  correspond 
ing  divisions :  /Science  of  Organized  Matter,  or  PHYSIOLOGY, 
and  Science  of  Unorganized  Matter,  or  GENERAL  PHYSICS. 

Physiology,  which  treats  of  the  laws  of  matter  as  modi 
fied  by  the  principle  of  vitality,  is  divided  into  two  principal 
branches :  Animal  Physiology,  or  ZOOLOGY,  and  Vegetable 
Physiology,  or  BOTANY.  Both  of  these  branches,  with  their 
various  subdivisions,  belong  to  the  domain  of  NATURAL 
HISTORY. 

All  unorganized  matter  may  be  divided  into  two  classes, 
Celestial  and  Terrestrial.  General  Physics  admits  of  two 
corresponding  divisions.  That  branch  which  treats  of  ce 
lestial  bodies,  including  the  earth  as  a  whole,  is  called  AS 
TRONOMY  ;  that  which  treats  of  terrestrial  bodies,  is  called 
TERRESTRIAL  PHYSICS. 

TERRESTRIAL  PHYSICS  is  again  subdivided  into  two 
branches.  The  first  is  called  Physics  Proper,  or  simply 
PHYSICS  ;  it  treats  of  the  general  properties  of  bodies.  The 
second  is  called  CHEMISTRY  ;  it  treats  of  the  nature  of  the 
ultimate  particles  of  bodies  and  of  their  laws  of  combination. 
The  first  of  these  branches,  or  PHYSICS,  is  the  subject  treated 
of  in  the  following  pages. 

Besides  the  branches  above  enumerated,  and  which  may 
be  called  Pure  Sciences,  there  are  others  that  depend  upon, 
or  are  applications  of,  two  or  more  of  them.  Such,  for  ex 
ample,  are  the  sciences  of  GEOLOGY,  MINERALOGY,  PHYSICAL 
GEOGRAPHY,  &c.  These  are  called  Mixed  Sciences. 


Into  what  may  Natural  Philosophy  be  divided  ?  What  is  Physiology,  and  what  are 
its  branches?  How  may  Unorganized  Matter  be  divided?  What  are.  the  cor 
responding  divisions  of  General  Physics?  Define  them.  How  is  Terrestrial  Phy 
sics  divided?  What  is  Physics  Proper ?  Chemistry?  What  are  the  Pure  Sciences, 
and  what  are  some  of  the  Mixed  Sciences  ? 


CHAPTER    I. 

PRELIMINARY    PRINCIPLES    AND   MECHANICS    OF   SOLIDS. 

I. — DEFINITIONS    AND     GENERAL     PROPERTIES    OF     MATTER. 

Definition  of  Physics— Physical  Agents. 

1.  PHYSICS  is  that  branch  of  Natural  Philosophy  which 
treats  of  the  general  properties  of  bodies,  and  of  the  causes 
that  modify  these  properties. 

The  principal  causes  that  modify  the  properties  of 
bodies  are:  Gravitation,  Heat,  Light,  Magnetism,  and 
Electricity.  These  causes  are  called  Physical  Agents. 

Definition  of  a  Body. 

2.  A  BODY  is  a  collection  of  material  particles ;    as  a 
stone,  or  a  block  of  wood.     A  body  which  is  exceedingly 
small  is  called  a  Material  Point. 

Bodies  are  made  up  of  small  particles,  called  Molecules, 
and  these  again  are  composed  of  still  smaller  elements,  called 
Atoms.  These  atoms  are  inconceivably  small,  and  are  held 
in  their  places  by  the  action  of  two  opposing  systems  of 
forces,  called  Molecular  Forces.  Those  which  tend  to  draw 
atoms  together  are  called  Attractive  Forces,  and  those 
which  tend  to  push  them  asunder  are  called  Repellent 
Forces.  Heat  is  the  principal  if  not  the  only  repellent 
>:brce  in  Nature. 

(1.)  What  is  Physics ?  "What  are  Physical  Agents?  Name  them.  (2.)  Define 
a  Body.  A  Material  Point.  An  Atom.  A  Molecule.  What  are  Molecular  Forces? 
Define  Attractive  and  Repellent  Forces. 


12  POPULAR  PHYSICS. 

Mass  and  Density. 

3.  The  MASS  of  a  body  is  the  quantity  of  matter  which 
it  contains. 

Different  bodies,  having  the  same  volume,  contain  very  different 
quantities  of  matter ;  for  example,  a  cubic  inch  of  lead  contains 
nearly  eleven  times  as  much  matter  as  a  cubic  inch  of  water.  Tli3 
masses  of  bodies  are  proportional  to  their  weights. 

The  DENSITY  of  a  body  is  the  degree  of  closeness  of 
its  particles. 

Those  bodies  in  which  the  particles  are  close  together  are 
said  to  be  dense  /  thus,  platinum  and  mercury  are  dense 
bodies.  Those  in  which  the  particles  are  not  close  together 
are  said  to  be  rare  ;  thus,  steam  and  air  are  rare  bodies. 
The  densities  of  bodies  having  the  same  bulk  are  propor 
tional  to  their  weights. 

Classification  of  Bodies. 

4.  Bodies  may  exist  in  two  different  states,  the  solid 
and  the  fluid. 

SOLIDS  are  those  which  tend  to  retain  a  permanent  form ; 
as  stones,  metals,  and  the  like.  The  particles  of  such  bodies 
adhere  to  each  other  with  considerable  energy,  and  this  ad 
hesion  can  be  overcome  only  by  the  exertion  of  some  effort. 

FLUIDS  are  those  whose  particles  move  freely  amongst 
each  other ;  as  water,  alcohol,  and  air.  Such  bodies  have 
no  tendency  to  retain  a  permanent  form,  but  assume  at 
once  the  form  of  the  containing  vessel. 

Fluids  are  divided  into  Liquids  and  Gases  or  Vapors* 
Liquids  arc  sensibly  incompressible ;  as  water,  wine,  and 
milk.  Gases  and  vapors  are  highly  compressible ;  as  at 
mospheric  air  and  steam. 


(3.)  What  is  the  mass  of  a  body?  Density?  Give  examples  of  dense  and  rare 
bodies.  (4.)  How  are  bodies  divided?  Define  solids  and  fluids.  How  are  fluids 
divided?  Define  liquids,  and  gases  OT  vapors. 


GENERAL   PROPERTIES    OF    BODIES.  13 

In  solids,  the  molecular  forces  of  attraction  are  greater 
than  the  repellent  forces,  hence  the  difficulty  of  separating 
their  molecules  ;  in  liquids,  the  attractive  and  repellent 
forces  are  sensibly  balanced  ;  in  gases,  the  repellent  are 
more  powerful  than  the  attractive  forces. 

Many  bodies  may  exist  in  each  of  the  three  states  in  succession. 
Thus,  if  ice  be  heated  until  the  repellent  forces  balance  those  of  at 
traction,  it  passes  into  the  liquid  state  and  becomes  water:  if  still 
more  heat  be  applied,  the  repellent  forces  prevail  over  those  of  at- 
ij  and  it  passes  into  the  state  of  vapor  and  becomes  steam. 


General  Properties  of  Bodies. 

5.  All  bodies  possess  certain  properties,  the  most  im 
portant  of  which  are  :  Magnitude,  Form,  Impenetrability, 
Inertia,   Porosity,    Divisibility,    Compressibility,   Dilata- 
bility,  and  Elasticity. 

Magnitude  and  Form. 

6.  The  MAGNITUDE  of  a  body  is  its  bulk,  or  the  por 
tion  of  space  that  it  fills.     It  is  evident  that  a  body  can  not 
exist  without    possessing   the   three   attributes   of  length, 
breadth,  and  thickness. 

The  FORM  of  a  body  is  its  external  shape.  Bodies  may 
have  the  same  magnitude  and  be  very  different  in  shape  ; 
they  may  likewise  be  of  the  same  form  and  yet  be  of  very 
different  magnitudes. 

Impenetrability. 

7.  IMPENETRABILITY  is  that  property  by  virtue  of  which 
no  two  bodies  can  occupy  the  same  place  at  the  same  time. 
This  property  is  self-evident,  although  phenomena  are  ob 
served  which  would  seem  to  conflict  with  it.     Thus,  when 
a  pint  of  alcohol  is  mixed  with  a  pint  of  water,  the  volume 
of  the  resulting  mixture  is  less  than  a  quart.     This  diminu- 


Illnstrate.    (  5.)  What  properties  belong  to  all  bodies  ?    (  6.)  What  is  Magnitude  ? 
Form?    (  1.)  What  is  Impenetrability?    Illustrate. 


14:  POPULAR   PHYSICS. 

tion  of  volume  arises  from  the  particles  of  one  of  the  fluids 
insinuating  themselves  between  those  of  the  other ;  but  it 
is  clear  that  where  a  particle  of  alcohol  is,  there  a  particle 
of  water  can  not  be.  In  like  manner  when  a  nail  is  driven 
into  a  board,  the  particles  of  the  latter  are  thrust  aside  and 
c  ompressed  to  make  room  for  those  of  the  former. 

Inertia. 

8.  INERTIA  is  the  tendency  which  a  body  has  to  main 
tain  its  state  of  rest  or  motion.  If  a  body  is  at  rest  it  has 
no  power  to  set  itself  in  motion,  or  if  it  is  in  motion  it  has 
no  power  to  change  either  its  rate  of  motion  or  the  direc 
tion  in  which  it  is  moving.  Hence,  if  a  body  is  at  rest,  it 
will  remain  at  rest,  or  if  in  motion,  it  will  move  on  uni 
formly  in  a  straight  line  until  acted  upon  by  some  force. 

The  reason  why  we  do  not  see  bodies  continue  to  move  on  uni 
formly  in  straight  lines,  when  set  in  motion,  is  that  they  are  con 
tinually  acted  upon  by  forces  which  change  their  state  of  motion^ 
Thus,  a  ball  thrown  from  the  hand,  besides  meeting  with  the  resist 
ance  of  the  air,  is  continually  drawn  downwards  by  the  attraction  of 
the  earth,  till  at  last  it  is  brought  to  rest. 

Many  familiar  phenomena  are  explained  by  the  principle  of  in 
ertia.  For  example,  when  a  vehicle  in  motion  is  suddenly  arrested, 
loose  articles  in  it  are  thrown  to  the  front,  because  they  tend  to  keep 
the  motion  which  they  had  acquired.  When  a  man  in  running 
strikes  his  foot  against  an  obstacle,  the  inertia  of  the  upper  part  of 
his  body  carries  it  forward,  and  he  falls  to  the  ground.  For  the 
same  reason,  when  a  man  jumps  from  a  car  in  motion,  he  will  be  in 
danger  of  falling  in  the  direction  of  the  moving  car.  It  is  inertia 
which  renders  accidents  upon  railroads  so  terrible.  When  from  any 
cause  the  locomotive  is  suddenly  arrested,  the  inertia  of  the  entire 
train  acts  to  pile  the  cars  together  in  one  general  wreck.  It  is  the 
inertia  of  the  hammer  that  enables  it  to  overcome  the  resistance 


Give  examples  of  apparent  penetrability.  (  8. >  What  is  Inertia?  Illustrate.  Why 
do  we  not  see  bodies  conform  to  the  law  of  inertia  t  Give  examples  of  the  prin 
ciple  of  inertia. 


GENERAL    PROPERTIES    OF    BODIES. 


15 


which  the  wood  offers  to  the  entering  nail;  and  in  driving  piles,  the 
principal  effect  is  due  to  the  inertia  of  the  descending  ram. 

Porosity. 

9.  POROSITY  is  the  degree  of  separation  between  the  mole- 
o  iles  of  a  body  The  inter 
vals  between  the  mole 
cules  are  called  pores. 
When  these  intervals  are 
very  great,  the  body  is 
said  to  be  porous,  as  in 
steam,  air,  and  gases. 
When  the  intervals  are 
very  small,  the  body  is 
said  to  be  dense,  as  in 
gold,  platinum,  and  mer 
cury.  Pores  must  not  be 
confounded  with  cells,  as 
in  sponge,  light  bread,  and 
the  like. 

All  bodies  are  more  or 
less  porous. 

The  following  experiment 
shows  the  porosity  of  leather. 
A  long  glass  tube  (Fig.  1)  is 
surmounted  by  a  brass  cup, 
with  a  thick  leather  bottom, 
fitting  the  tube  air-tight.  The 
lower  end  of  the  tube  termin 
ates  in  a  brass  cap,  which  is 
attached  to  a  machine  for  ex 
hausting  the  air  from  the  tube, 
called  an  air-pump. 

If  a  quantity  of  mercury  is 


(9.)  What  is  Porosity?    When  are  bodies  porous?    When  dense?    Explain  the 
experiment  showing  the  porosity  of  leather. 


16 


POPULAR   PHYSICS. 


poured  into  the  upper  cup,  and  the  air  exhausted  from  the  tube,  the 
mercury,  being  pressed  down  by  the  external  air,  is  seen  falling 
through  the  leather  in  small  drops  like  rain. 

Gold  was  shown  to  be  porous  by  some  Florentine  philosophers  in 
the  following  manner.  A  hollow  sphere  of  gold  was  filled  with 
water  and  tightly  closed,  after  which  it  was  subjected  to  great  pres 
sure.  The  water  was  seen  to  issue  from  the  globe  and  form  on  its 
surface  like  dew.  The  experiment  has  since  been  repeated  witli 
other  metals,  and  with  like  results. 

Gases  are  shown  to  be  porous  by  their  enormous  reduction  in  volume 
when  compressed:  if  a  gas  be  introduced  into  ajar,  it  will  spread  by 
its  expansive  force  and  completely  fill  the  vessel ;  if  a  second  gas  be 
introduced  into  the  same  vessel,  it  likewise  expands  and  fills  the 
vessel  as  though  the  first  gas  did  not  exist.  This  proves  that,  the 
molecules  of  the  second  gas  arrange  themselves  in  the  pores  of  the 
first. 


Fig.  8. 


The  property  of  porosity  finds  an  important  application  in 
the  process  of  filtering,  that  is,  in  separating  foreign  particles 
from  liquids. 


n  the  Florentine  experiment.    What  are  filters? 


GENERAL   PROPERTIES    OF   BODIES.  17 

Fig.  2  represents  a  filter  for  purifying  water ;  it  is  simply  a  box 
divided  into  two  parts  by  a  partition  of  porous  stone,  A.  The  water 
to  be  filtered  is  placed  in  the  upper  part,  from  which  it  passes  slowly 
into  the  lower  part  through  the  pores  of  the  stone.  In  one  corner  of 
the  box  is  a  tube,  a.  which  permits  the  air  to  escape  as  the  lower 
part  of  the  box  fills  with  water.  The  purified  water  is  drawn  off  by 
means  of  a  faucet  near  the  bottom  of  the  box. 

Fig.  3  represents  a  filter  used  by  chemists.  It  consists  of  a  pocket 
of  some  porous  material,  as  felt,  for  example,  suspended  by  cords. 
The  substance  to  be  filtered  is  poured  into  the  pocket,  from  which 
the  liquid  escapes  slowly  through  the  pores,  leaving  the  solid  parts 
behind. 

Filters  are  also  formed  by  layers  of  powdered  charcoal,  or  finely 
ground  quartz,  through  the  pores  of  which  the  liquids  pass.  It  is  to 
a  natural  filtration  through  sand  that  many  kinds  of  spring  water 
owe  their  purity. 

It  is  in  consequence  of  porosity,  that  burning  coals  covered  up  with 
ashes  continue  to  burn  slowly.  The  air  which  is  necessary  to  com 
bustion  penetrates  through  the  pores  of  the  ashes,  in  sufficient  quan 
tity  to  keep  the  fire  from  being  entirely  extinguished. 

Finally,  it  is  in  consequence  of  their  porosity,  that  many  kinds  of 
wood  absorb  moisture  from  the  air.  and  tend  to  swell  and  crack  ;  this 
difficulty  is  remedied  by  applying  oils  and  varnishes,  which  close  the 
pores  and  exclude  the  moisture. 

Divisibility. 

1O.  DIVISIBILITY  is  that  property  by  virtue  of  which  a 
body  may  be  divided  into  parts.  All  bodies  are  capable  of 
subdivision,  and  in  many  cases  the  parts  that  may  be  ob 
tained  are  of  almost  inconceivable  minuteness. 

The  following  examples  serve  to  show  the  extreme  smallness  of 
the  molecules  of  matter.  A  single  grain  of  carmine  imparts  a  sen 
sible  color  to  a  gallon  of  water  :  this  gallon  of  water  may  be  sepa 
rated  into  a  million  of  drops,  and  if  we  suppose  each  drop  to  contain 
ten  particles  of  carmine,  which  is  a  low  estimate,  we  shall  have 


Explain  the,  water  filter-.    Explain  the  chemist'*  filter.     Other  applications  of 
porosity.    (10.)  What  is  Divisibility  ?     Give  examples  of  divisibility  by  solution. 


18  POPULAR  PHYSICS. 

divided  the  grain  of  carmine  into  ten  million  of  molecules,  each  of 
which  is  visible  to  the  naked  eye. 

The  microscope  reveals  to  us,  in  certain  vegetable  infusions,  ani 
malcule  so  small  that  several  hundred  of  them  can  swim  in  a  drop 
of  water  that  adheres  to  the  point  of  a  needle.  These  little  animals 
are  capable  of  motion,  and  even  of  preying  upon  each  other :  they  there 
fore  possess  organs  of  motion,  digestion,  and  the  like.  How  minute, 
then,  must  be  the  molecules  which  go  to  make  up  these  organs. 

A  grain  of  musk  is  capable  of  diffusing  its  odor  through  an  apart 
ment  for  years,  with  scarcely  an  appreciable  diminution  of  its  weight. 
This  shows  that  the  molecules  of  musk  continually  given  off  to  re 
plenish  the  odor,  are  of  inconceivable  srnallness. 

The  blood  of  animals  consists  of  minute  red  globules  swimming 
in  a  serous  fluid  :  these  globules  are  so  small  that  a  drop  of  human 
blood,  no  larger  than  the  head  of  a  small  pin,  contains  at  least 
50.000  of  them.  In  many  animals  these  globules  are  still  smaller  ; 
in  the  musk  deer,  for  example,  a  single  drop  of  blood  of  the  size  of  a 
pin's  head  contains  at  least  a  million  of  them. 

Compressibility. 

11.  COMPRESSIBILITY  is  the  property  of  being  reduced 
to  a  smaller  space  by  pressure.  This  property  is  a  conse 
quence  of  porosity,  and  the  change  of  bulk  comes  from  the 
particles  being  brought  nearer  together  by  the  pressure. 
Sponge,  india-rubber,  cork,  and  elder  pith,  are  examples 
of  compressible  bodies  ;  they  may  be  sensibly  diminished  in 
volume  by  the  pressure  of  the  fingers.  Gases  are,  however, 
the  best  examples  of  compressible  bodies. 

Fig.  4  represents  an  apparatus  by  means  of  which  the  com 
pressibility  of  gases  may  be  shown.  It  consists  of  a  tube  of  glass, 
with  metallic  caps,  completely  closed  at  its  lower  end.  An  air-tight 
piston  is  introduced  at  the  upper  end,  and  on  being  pushed  down  we 
see  the  inclosed  air  reduced  to  the  half,  fourth,  and  even  the  hun 
dredth  part  of  its  original  bulk. 


Examples  of  minute  animals.    Examples  of  odoriferous  bodies.    Blood  globules. 
(11.)  What  is  Compressibility  ?     Examples.    Explain  the  experiment. 


GENERAL   PROPERTIES    OF   BODIES.  19 

Liquids  are  only  slightly  compressible,  nevertheless  nice 
experiments  show  that  even  they  can  be  somewhat  reduced 
in  bulk  by  pressure. 


Fig  4. 

Metals  are  compressible,  as  is  shown  in  the  process  of 
stamping  coins,  metals,  and  the  like. 

Dilatability. 

12.     DILATABILITY  is  the  property  that  a  body  possesses 
of  assuming  a  greater  bulk  under  certain  circumstances. 
In  the  experiment  upon  air,  explained  in  the  last  article, 


Are  liquids  compressible  ?    Are  metals  compressible  ?    How  shown  ?    ( 1 2 . )  What 
is  Dilatability  ? 


20  POPULAR     PHYSICS. 

if  the  piston  be  raised  after  the  air  has  been  compressed,  it 
will  expand  and  fill  the  tube.  Almost  all  bodies  expand  on 
being  heated.  It  is  on  this  principle  that  thermometers  are 
constructed.  In  cooling,  bodies  contract. 

A  familiar  example  of  dilatability  and  contractibility  is  shown  in 
the  process  of  fitting  the  tire  upon  a  carriage  wheel.  The  tire  is 
made  a. little  smaller  than  the  wheel,  but  on  being  heated  it  expands 
so  as  to  embrace  it :  on  cooling  it  contracts  again  and  draws  the 
parts  of  the  wheel  tightly  together. 

The  same  property  of  metals  has  been  used  for  producing  great 
pressures,  and  even  for  restoring  inclined  walls  to  an  erect  position. 

Elasticity. 

13.  ELASTICITY  is  the  property  which  bodies  possess  of 
recovering  their  original  shape  and  size  after  having  been 
either  compressed  or  extended. 

Bodies  differ  in  their  degree  of  elasticity,  yet  all  are  more 
or  less  elastic.  India-rubber,  ivory,  and  whalebone  are 
examples  of  highly  elastic  bodies.  Putty  and  clay  are 
examples  of  those  which  are  only  slightly  elastic. 

If  air  be  compressed,  its  elasticity  tends  to  restore  it  to  its  original 
bulk;  this  property  has  been  utilized  in  making  air-beds,  air-cush 
ions,  and  even  in  forming  car-springs.  If  a  spring  of  steel  be  bent. 
its  elasticity  tends  to  unbend  it;  this  principle  is  employed  in  giving 
motion  to  watches,  clocks,  and  the  like.  If  a  body  be  twisted,  its 
elasticity  tends  to  untwist  it,  as  is  observed  in  the  tendency  of  yarn 
and  thread  to  untwist ;  this  principle,  under  the  name  of  torsion^  is 
used  to  measure  the  deflective  force  of  magnetism.  If  a  body  be 
stretched,  its  elasticity  tends  to  reduce  it  to  its  original  length,  as  is 
shown  by  stretching  a  piece  of  india-rubber,  and  then  allowing  it 
to  contract. 

We  see  that  the  elasticity  of  a  body  may  be  brought  into  play  by 
four  different  methods  :  by  -pressure,  lay  flexure  or  bending,  by  lorsion 

Example.  Application  in  putting  tire  upon  a  icJieel.  Example  of  restoring 
wall*.  (13.)  What  is  Elasticity?  Give  examples  of  highly  and  slightly  elastic 
bodies.  Give  examples  of  the  applications  of  elasticity.  How  may  elasticity  to 
brought  inio  play  ?  Examples. 


GENERAL   PROPERTIES    OF   BODIES. 


or  twisting,  and  by  tension  or  stretching.  In  whatever  way  it  may 
be  developed,  it  is  the  result  of  molecular  displacement.  Thus, 
when  air  is  compressed,  the  repulsions  between  the  molecules  tend 
to  expand  it.  Again,  when  a  spring  is  bent,  the  particles  on  the  out 
side  are  drawn  asunder,  whilst  those  on  the  inside  are  pressed  to, 
gether;  the  attractions  of  the  former  and  the  repulsions  of  the  latter 
tend  to  restore  the  spring  to  its  original  shape. 

The  most  elastic  bodies  are  gases  :  after  them  come  tempered  steel, 
whalebone,  india-rubber,  ivory,  glass.  &c. 

Fig.  5  illustrates  the  meth 
od  of  showing  that  ivory 
is  elastic,  and  at  the  same 
time  that  the  cause  of  its 
elasticity  is  molecular  dis 
placement.  It  consists  of  a 
polished  plate  of  marble,  over 
which  is  spread  a  thin  layer 
of  oil.  If  a  ball  of  ivory  be 
let  fall  upon  it  from  different 
heights,  it  will  at  each  time 
rebound,  leaving  a  circular 
impression  on  the  plate,  which 
is  the  larger  as  the  ball  falls 
from  a  greater  height.  This 
experiment  shows  that  the 
ball  is  flattened  each  time  by 

the  fall,  that  the  flattening  in-  Fig.  5. 

creases  as  the  height  increases, 
and  that  the  action  of  the  compressed  molecules  causes  it  to  rebound. 

The  property  of  elasticity  is  utilized  in  the  arts  in  a  great  variety 
of  ways.  When  a  cork  is  forced  into  the  mouth  of  a  bottle,  its  elas 
ticity  causes  it  to  expand  and  fill  the  neck  so  as  to  render  it  both 
water  and  air-tight.  It  is  the  elasticity  of  air  that  causes  india- 
rubber  balls,  filled  with  air,  to  rebound  when  thrown  upon  hard  sub 
stances.  It  is  the  elasticity  of  steel  that  renders  it  of  use  in 


What  bodies  are  most  elastic  ?  How  is  it  shtncn  that  ivory  is  elastic?  Explain 
the  experiment.  Explain  sows  of  the  'applications  of  elasticity.  Corking  lottle» 
Springs. 


22  POPULAR     PHYSICS. 

springs  for  moving  machinery,  as  well  as  for  easing  the  motion  of 
carriages  over  rough  roads.  It  is  the  elasticity  of  cords  that  renders 
them  applicable  to  musical  instruments.  It  is  the  elasticity  of  air 
that  renders  it  a  fit  vehicle  for  transmitting  sound.  It  is  the  elas 
ticity  of  the  etherial  medium  pervading  space  that  renders  it 
capable  of  transmitting  light. 


II.  —  MECHANICAL       PRINCIPLES 

Definition   of  Mechanics. 

14.  MECHANICS  is  that  branch  of  Physics  which  treats 
of  the  laws  of  rest  and  motion.     It  also  treats  of  the  action 
offerees  upon  bodies. 

Rest  and  Motion. 

15.  A  body  is  at  BEST  when  it  retains  its  position  in 
space.     It  is  in  MOTION  when  it  continually  changes  its  po 
sition  in  space. 

A  body  is  at  rest  with  respect  to  surrounding  bodies, 
when  it  retains  the  same  relative  position  with  respect  to 
them,  and  it  is  in  motion  with  respect  to  surrounding  ob 
jects  when  it  continually  changes  its  relative  position  with 
respect  to  them.  These  states  of  rest  and  motion  are  called 
Relative  Rest  and  Relative  Motion,  to  distinguish  them 
from  Absolute  Rest  and  Absolute  Motion. 

When  a  body  remains  fixed  on  the  deck  of  a  moving  vessel  or  boat, 
it  is  at  rest  with  respect  to  the  parts  of  the  vessel,  although  it  par 
takes  with  them  in  the  common  motion  of  the  vessel.  When  a  man 
walks  about  the  deck  of  a  vessel,  he  is  in  motion  with  respect  to  the 
parts  of  the  vessel,  but  he  may  be  at  rest  with  respoct  to  objects  on 
shore :  this  will  be  the  case  when  he  travels  as  fast  as  the  vessel 
sails,  but  in  an  opposite  direction.  In  consequence  of  the  earth's 
motion  around  its  axis  and  about  the  sun.  together  with  the  motion 


Stringed  instruments.  Transmission  of  light.  (14.)  What  is  Mechanics? 
(15.)  When  is  a  body  at  rest?  When  in  motion?  Explain  relative  and  absolute 
rest  and  motion.  lUwtrate  by  examples. 


MECHANICAL   PRINCIPLES.  23 

of  the  whole  solar  system  through  space,  it  is  not  likely  that  any 
part  of  our  system  is  in  a  state  of  absolute  rest  for  any  appreciable 
length  of  time. 

Different  kinds    of  Motion. 

16.  MOTION  may  be  rectilinear  or  curvilinear  /    it  is 
rectilinear  when  the  path  of  the  moving  body  is  a  straight 
line,  and  it  is  curvilinear  when  this  path  is  a  curved  line. 
The  motion  of  a  train  of  cars  along  a  straight  track  is  an 
example  of  rectilinear  motion ;  the  motion  of  the  same  train 
in   passing   round   a   curve   is   an   example   of  curvilinear 
motion. 

Uniform  Motion— Velocity. 

17.  UNIFORM  MOTION  is  that  in  which  a  body  passes 
over  equal  spaces  in  equal  times.     Thus,  every  point  on  the 
surface  of  the  earth  is,  by  its  revolution,  carried  around  the 
axis  \yith  a  uniform  motion. 

In  this  kind  of  motion  the  space  passed  over  in  one 
second  of  time  is  called  the  velocity.  Thus,  if  a  train  of 
cars  travel  uniformly  at  the  rate  of  20  miles  per  hour,  its 
velocity  if.  29.3  feet.  Instead  of  taking  a  second  as  the  unit 
of  tiniL\  we  might  adopt  a  minute,  or  an  hour.  In  the  same 
case  as  before  we  might  say,  that  the  velocity  of  the  train 
is  one  thi-d  of  a  mile  per  minute,  or  twenty  miles  per  hour. 

Varied  Motion— Accelerated  and  Retarded  Motion. 

18.  VARIED  MOTION   is  that   in  which   a  body  passes 
over  unequal  spaces  in  equal  times.     If  the  spaces  passed 
over  in  equal  times  go  on  increasing,  the  motion  is  acceler 
ated  ;  such  is  the  motion  of  a  train  of  cars  when  starting, 
or  that  of  a  body  falling  towards  the  surface  of  the  earth. 
If  the  spaces  passed  over  go  on  decreasing,  the  motion  is 


(  1 6.1  What  is  Rectilinear  Motion  ?    Curvilinear  Motion  ?    Examples. 

(17)  What  is  Uniform  Motion?  Example.  What  is  meant  by  velocity  ?  Exampla. 

(  18.)  What  is  Varied  Motion  ?    When  is  it  accelerated  and  vajn  retard  ed  ? 


21  POPULAR     PHYSICS. 

retarded  /  such  is  the  motion  of  a  train  of  cars  when  coming 
to  rest,  or  that  of  a  body  thrown  vertically  upwards. 

When  the  spaces  passed  over  in  equal  times  are  continu 
ally  increased  or  decreased  by  the  same  quantity,  the  motion 
is  uniformly  accelerated,  or  uniformly  retarded.  The  mo- 
tion  of  a  body  falling  in  a  vacuum,  is  uniformly  acceler* 
ated ;  that  of  a  body  shot  vertically  upwards  in  a  vacuum, 
is  uniformly  retarded. 

The  velocity  of  a  body  having  varied  motion  at  any  time, 
is  the  rate  of  the  body's  motion  at  that  time.  In  varied 
motion  the  velocity  is  continually  changing. 

Forces,  Powers,    and  Resistances. 

19.  If  a  body  is  at  rest,  any  cause  that  tends  to  set  it 
in  motion,  is  called  a  Force  ;  if  a  body  is  in  motion,  any 
cause  that  tends  to  make  it  move  faster,  or  slower,  or  to 
change  its  direction,  is  called  a  Force. 

A  Fores,  then,  is  any  cause  that  tends  to  change  the 
state  of  a  body,  with  respect  to  rest  or  motion. 

The  attractions  and  repulsions  between  the  molecules  of  bodies 
are  forces  ;  the  muscular  efforts  of  men  or  animals,  employed  in 
accomplishing  any  kind  of  work,  are  forces;  the  elastic  efforts  of 
gases  and  vapors  are  forces. 

Forces  which  act  to  produce  motion  are  called  Powers  j 
those  which  act  to  prevent  or  destroy  motion  are  called 
Resistances.  The  effort  of  steam  employed  in  moving  a 
train  of  cars  is  a  power,  whilst  friction  and  the  inertia  of  the 
air,  which  tend  to  retard  the  motion,  are  resistances.  Pow 
ers  tend  to  accelerate  motion,  and  are  for  that  reason  called 
Accelerating  Forces.  Resistances,  on  the  contrary,  tend  to 
retard  motion,  and  are  for  that  reason  called  Retarding 
Forces. 


Examples.  Define  uniformly  accelerated  and  uniformly  retarded  motion.  Ex 
amples.  (19.)  What  is  a  Force?  Examples.  Define  Powers  and  Resistances. 
Examples.  By  what  other  names  may  they  be  called  ? 


MECHANICAL   PRINCIPLES.  25 


Distinctive   Characteristics  of  Forces. 

2O.  In  order  that  the  effect  of  any  force  may  be  com 
pletely  understood,  three  things  must  be  known :  its  point 
of  application,  its  direction,  and  its  intensity. 

The  point  of  application  of  a  force  is  the  point  where  it 
exerts  its  action.  Thus,  in  Fig.  6,  which  represents  a  child 
drawing  a  wagon,  the  force  exerted  by  the  child  has  its 
point  of  application  at  A. 


Fig.  6. 


The  direction  of  a  force  is  the  line  along  which  it  acts ; 
thus,  in  Fig.  6,  the  line  AB  is  the  direction  of  the  force 
exerted  by  the  child. 

The  intensity  of  a  force  is  the  energy  with  which  it  acts ; 
thus,  in  the  same  example  as  before,  the  intensity  of  the 
force  exerted  is  the  energy  which  the  child  exerts  in  over 
coming  the  resistance  of  the  wagon. 

The  intensity  of  a  force  is  measured  in  pounds ;  thus,  a 
force  of  fifty  pounds  is  a  force  necessary  to  sustain  a  weight 
of  fifty  pounds.  The  intensity  of  a  force  may  be  represented 
by  a  distance  which  is  usually  laid  off  on  the  line  of  direc- 

(  20  )  What  three  elements  determine  a  force  ?  Define  the  point  of  application. 
The  line  of  direction.  The  intensity.  How  is  the  intensity  measured  ?  How  repre 
sented  ?  Example. 


26  POPULAR     PHYSICS. 

tion  of  the  force.  Having  assumed  some  unit  of  length, 
say  one  tenth  of  an  inch,  to  represent  one  pound,  this  is  set 
off  as  many  times  as  the  force  contains  pounds,  in  the 
example  taken,  if  we  suppose  the  force  exerted  to  be  seven 
pounds,  and  lay  off  from  A  to  C  seven  tenths  of  an  inch, 
then  will  A  C  represent  the  force  both  in  direction  and  in 
tensity. 

Resultant  and   Component   Forces. 

21.  When  a  body  is  solicited  by  a  single  force,  it  is  evi 
dent,  if  no  obstacle  intervene,  that  it  will  move  in  the  direc 
tion  of  that  force ;  but  if  it  is  solicited  at  the  same  time  by 


Fig.  7. 


several  forces  acting  in  different  directions,  it  will  not,  in 
general,  move  in  the  direction  of  any  one  of  them.  For 
example,  if  two  men  on  opposite  sides  of  a  river  tow  a  boat 
by  means  of  a  rope,  as  represented  in  Fig.  V,  the  boat  will 
not  move  either  in  the  direction  AB,  or  A  C,  but  it  will 
move  in  some  intermediate  direction,  as  AE\  that  is,  it  will 
advance  as  though  it  were  solicited  by  a  single  force  di 
rected  from  A  towards  E.  This  single  force,  which  would 
produce  the  same  effect  as  the  two  separate  forces,  is  called 

(21.)  What  is  a  Eesultant  of  several  forces  ? 


MECHANICAL   PRINCIPLES.  %  I 

then-  Resultant.      The  separate  forces  are  called  Compo- 
nents  of  the  resultant/ 

In  eeneral  the  resultant  of  any  number  of  forces  is  a  single 
force  whose  effect  is  equivalent  to  that  of  the  whole  group. 
The  individual  forces  of  the  group  are  called '  Components. 
Parallelogram  of  Forces. 

22      It  -is  shown  in  Mechanics  (Peck's  Mechanics,  Art. 
25),  that  \£AB  and  AD,  Fig.  8,  represent  two  forces  acting 
at  J.,  their  resultant  will 
be  represented  by  AC. 
That   is,    if  two  forces 


Fig.  8. 


are  represented  in  direc 
tion  and  intensity  by  the 
adjacent  sides  of  a  par- 
allelogram,  their  result 
ant  will  be  represented  in 

direction    and  intensity   In]   that   diagonal  which  passes 
through  their  point  of  intersection. 

This  principle  is 
called  the  Parallelo 
gram  of  Forces. 

The  operation  of 
finding  the  result 
ant  when  the  com 
ponents  are  given 
is  called  Composi 
tion  of  Forces  ;  the 
reverse  operation  is 
called  Resolution 
of  Forces. 

When  two  forces 
are  applied  at  the 
same  point,  as  shown  in  Fi 


Fie:.  9. 

9,  we  lay  off  distances  AE 


What  are  Components?    Illustrate.    (  22.)  Enunciate  the  parallelogram  of  forces. 


28  POPULAR    PHYSICS. 

and  AD  to  represent  the  forces,  and  having  completed  the 
parallelogram,  we  draw  its  diagonal  A  C ;  this  will  be  their 
resultant.  If  the  resultant  A  C  is  known,  and  the  directions 
of  its  components  are  given,  we  draw  through  C  the  lines 
CD  and  CB  parallel  to  their  directions  ;  then  will  the  in 
tercepted  lines  AD  and  AB  be  components  of  the  force 
AC. 

Practical  Example  of  Composition  of  Forces. 

23.     A  bird,  in  flying,  strikes  the  air  with  both  wings, 
and  the  latter  offers  a  resistance  which  propels  him  forward. 


Let  AK  and  AH,  in  Fig.  10,  represent  these  resistances. 
Draw  AB  and  AD  equal  to  each  other,  and  complete  the 
parallelogram  AC ';  draw  also  the  diagonal  AC.  Then 
will  A  C  represent  the  resultant  of  the  two  forces,  and  the 
bird  will  move  exactly  as  though  impelled  by  the  single 
force  CA. 


How  is  the  resultant  found  when  the  components  are  known  ?        How  are  the 
components  found  ?    ( 23.)    Explain  the  flight  of  a  bird. 


MECHANICAL   PKIJX'CIPLES. 


Practical  Example    of  Resolution  of  Forces. 

24.  When  a  sail-boat  is  propelled  by  a  breeze  acting  on 
the  quarter  in  the  direction  va  (Fig.  11),  we  may,  by  the 
rule  in  Art.  22,  resolve  the  intensity  of  the  wind  into  two 
components,  one,  ca,  in  the  direction  of  the  keel,  and  the 


Fig.  11. 

other,  ba,  at  right  angles  to  it.  The  first  component  alone 
is  effective  in  giving  a  forward  motion  to  the  boat,  whilst 
the  second  is  partly  destroyed  by  the  resistance  which  the 
water  offers  to  the  keel,  and  partly  employed  in  giving  a 
lateral  motion  to  the  boat.  This  lateral  motion  is  called' 
leeway. 

Resultant  of  Para!?  el  Forces. 

25.  When  two  forces  act  in  the  same  direction,  as  when 
two  horses  pull  at  the  ends  of  a  whiffle-tree  to  draw  a 
wagon,  their  resultant  is  equal  to  the  sum  of  the  forces. 
When  they  act  in  a  contrary  direction,  as  in  the  case  of  a 
steamboat  ascending  a  river,  where  the  force  of  the  engine 
acts  to  propel  the  boat  forward,  whilst  the  current  acts  to 

( 24  )  Explain  the  sailing  of  a  boat.  (  25.)  What  is  the  resultant  of  parallel  forces 
when  they  act  in  the  same  direction?  When  they  act  in  opposite  directions?  Ex 
amples. 


30  POPULAR     PHYSICS. 

retard  its  progress,  their  resultant  is  equal  to  the  difference 
of  the  forces. 

Equilibrium  of  Forces. 

26      When  several  forces  acting  upon  a  body  exactly 
balance  each  other,  they  are  said  to  be  in  equilibrium. 


Fie.  12. 


The  simplest  case  of  equilibrium  is  that  of  two  equal 
forces  acting  against  each  other,  as  in  the  case  where  two 
men  of  equal  strength  pull  at  the  two  ends  of  a  rope,  as 
shown  in  Fig.  12. 

In  the  same  manner,  if  two  buckets  of  equal  weight  are 
suspended  in  a  well  from  the  ends  of  a  rope  passing  over  a 
pulley,  they  will  be  in  equilibrium. 

When  a  body  rests  upon  a  table,  there  is  an  equilibrium 
between  the  weight  of  the  body  which  urges  it  downwards, 
and  the  resistance  of  the  table  which  prevents  it  from  falling. 
If  the  weight  becomes  greater  than  the  resistance,  the  table 
breaks  and  the  body  falls. 

Centrifugal   and   Centripetal   Forces. 

27.  The  CENTRIFUGAL  FORCE  is  the  resistance  which 
a  body  offers  to  a  force  which  tends  to*  deflect  it  from  its 
course. 

In  consequence  of  its  inertia,  a  body  always  tends  to 
move  in  a  straight  line,  and  if  we  see  it  move  in  a  curved 
line  it  is  because  some  force  is  acting  to  turn  it  from  its  path. 
This  deflecting  force  is  called  the  Centripetal  Force,  and 


(26.)  When  are  forces  in  equilibrium?     Illustrate  by  examples.    (27.)  What  is 
the  Centrifugal  Force  ?    Centripetal  Force  ? 


MECHANICAL   PKINCIPLES. 


31 


because  action  and  reaction  are  always  equal,  the  centri 
petal  and  centrifugal  forces  are  always  opposed  and  equal 
to  each  otJier.  If  a  ball  is  whirled  about  the  hand,  being  re 
tained  by  a  string,  it  has  a  continual  tendency  to  fly  off, 
which  tendency  is  resisted  by  the  strength  of  the  string ;  the 
tendency  to  fly  off  is  due  to  the  centrifugal  force,  and  the 
force  which  resists  this  tendency  is  the  centripetal  force. 

The  curved  path  in  which  a  body  moves    may  be  regarded   as 
made  up  of  short  straight  lines,  and  if  at  any  instant  the  centripetal 


Fig.  13. 

force  were  destroyed,  the  body  would  continue  to  move  along  that 
line  on  which  it  was  situated  :  that  is,  its  new  path  would  be  tangent 
to  its  old  one. 

The  existence  of  the  centrifugal  force  may  be  shown  experiment- 
Example.     How  docs  a  body  move  when  the  centripetal  force  is  destroyed  f 


32  POPULAR    PHYSICS. 

ally  by  the  apparatus  represented  in  Fig.  13.  It  consists  of  a  bar, 
AB.  having  its  ends  bent  up  so  as  to  hold  a  wire  which  is  stretched 
between  them.  On  this  wire  two  ivory  balls  are  strung  so  as  to 
slide  along  it,  and  the  whole  bar  is  made  to  turn  about  an  axis  at 
right  angles  to  it  by  means  of  a  crank  and  two  bevelled  wheels. 
When  the  bar  is  made  to  revolve  about  the  axis,  the  balls,  acted 
upon  by  the  centrifugal  force,  are  thrown  against  the  ends  of  the 
bar  with  an  energy  which  becomes  greater  as  the  motion  of  revolu 
tion  becomes  more  rapid. 

Some  Effects   of  the  Centrifugal  Force. 

28.  When  a  train  of  cars  turns  round  a  curve  in  the 
road,  the  centrifugal  force  tends  to  throw  the  train  oft'  the 
track,  a  tendency  which  is  resisted  by  raising  the  outer  rail 
and  by  making  the  wheels  conical. 

It  is  in  consequence  of  the  centrifugal  force,  that  the  mud 
adhering  to  the  tire  of  a  carriage- wheel  is  thrown  off  in  all 
directions. 

In  the  circus,  where  horses  are  made  to  travel  rapidly 
around  in  a  curved  path,  the  centrifugal  force  tends  to  over 
turn  them  outwards,  which  tendency  is  partly  overcome  by 
making  the  outside  of  the  track  higher  than  the  inside,  and 
partly  by  both  horse  and  rider  inclining  inwards,  so  as  to 
make  the  resultant  of  their  weight  and  the  centrifugal  force 
perpendicular  to  the  path. 

When  a  sponge  filled  with  water  and  held  by  a  string  is 
whirled  rapidly  around,  the  centrifugal  force  throws  off  the 
water  and  leaves  the  sponge  dry.  This  principle  has  been 
used  for  drying  clothes  in  the  laundry. 

A  very  remarkable  effect  of  the  centrifugal  force  is  the 
flattening  of  our  earth  at  the  poles.  The  earth  turns  on  its 
axis  every  twenty-four  hours,  which  rotation  gives  rise  to  a 
centrifugal  force  at  every  point  of  its  surface.  At  the 


Explain  the  experiment.  (28)  Give  examples  of  the  action  of  the  centrifugal 
force.  Cars  on  a  curve.  Mud  from  wheel.  Circus.  Sponge.  Effect  on  the  form  of 
the  earth. 


MECHANICAL    PRINCIPLES. 


33 


equator  the  centrifugal  force  is  greatest,  because  the  ve 
locity  is  there  the  greatest,  and  from  the  equator  it  grows 
feebler  towards  each  pole,  where  it  is  zero.  The  centrifugal 
force  at  every  point  is  perpendicular  to  the  axis,  and  may  be 
resolved  into  two  components,  one  directed  outwards  from 
the  centre,  and  the  other  perpendicular  to  this.  The  former 
component  lessens  the  weight  of  bodies,  and  the  latter  acts 
to  heap  the  particles  up  towards  the  equator.  It  has  been 
found  that  the  earth  is  a  spheroid,  flattened  at  the  poles. 
The  polar  diameter  is  about  twenty-six  miles  shorter  than 
the  equatorial  diameter.  Observations  upon  the  heavenly 
bodies  show  that  other  planets  are  in  like  manner  flattened 
at  their  poles. 

The  manner  in  which  the  centrifugal  force  acts  to  flatten  a  sphere, 
is  shown  experimentally  by  an  apparatus, 
represented  in  Fig.  14.  This  apparatus 
consists  of  a  vertical  rod  to  which  a  mo 
tion  of  rotation  may  be  imparted,  as 
shown  in  Fig.  13.  At  the  lower  part  of 
this  rod  four  strips  of  brass  are  firmly 
fastened  and  bent  into  circles,  as  shown 
by  the  dotted  lines;  their  upper  ends 
are  fastened  to  a  ring  which  is  free 
to  slide  up  and  down  the  rod.  When 
the  axis  is  made  to  revolve  rapidly,  the 
centrifugal  force  causes  the  ring  to  slide 
down  the  rod.  the  hoops  become  more 
curved,  as  shown  in  the  figure,  and  the 
whole  assumes  the  appearance  of  a  flat 
tened  sphere. 

Machines. 

29.  A  MACHINE  is  any  contrivance  by  means  of  which 
a  force  acting  at  one  point  is  made  to  produce  an  effect  at 
some  other  point. 


Fig.  14. 


Effect  on  the  weight.    Explain  the  experiment.    (29.)  What  is  a  Machine  ? 

2* 


34  POPULAR    PHYSICS. 

--• > 

According  to  this  definition  every  tool  used  in  the  arts  is  a  machine ; 
in  common  language,  however,  the  term  is  only  applied  to  more 
complex  combinations.  In  this  sense,  a  machine  consists  of  a  col 
lection  of  moving  pieces  called  elements,  kept  in  position  by  a  frame. 
The  piece  to  which  the  motive  power  is  applied  is  called  the  recipi 
ent,  the  piece  that  performs  the  work  is  called  the  tool,  and  these, 
with  their  connecting  pieces,  make  up  a  train  of  mechanism.  The 
elements  of  a  train  are  called  Elementary  Macliines,  or  mechanical 
powers,  and  are  seven  in  number  (Art.  449). 

Of  these  the  lever*is  most  important  to  the  student  of  Physics. 
The  others  are  fully  discussed  in  Chapter  XI. 

The  Lever. 

3O.  A  LEVER  is  an  inflexible  bar  free  to  turn  about  a 
fixed  point,  called  the  Fulcrum,  and  acted' upon  by  two 
forces  which  tend  to  turn  it  in  opposite  directions.  The 
force  which  acts  as  a  motor,  is  called  the  Power^  the  other 
one  is  called  the  Resistance. 

Levers  are  of  three  classes,  according  to  the  position  of 
the  fulcrum  with  respect  to  the  power  and  resistance. 


Fig.  15. 

Lever  of  the  first  class. — In  this  class  the  fulcrum  is  be 
tween  the  power  and  the  resistance.  Such  a  lever  is  repre 
sented  in  Fig.  15.  The  hand  is  the  power,  the  weight  P  is 
the  resistance,  and  the  fixed  point  C  is  the  fulcrum. 

What  is  a  train  of  mechanism  ?  What  is  the  recipient  ?  Tool  ?  (3O.)  What  is 
a  lever  ?  How  many  classes  are  there  ?  Examples  of  each. 


MECHANICAL    PRINCIPLES. 


35 


Lever  of  the  second  class. — In  this  class  the  fulcrum  is 
beyond  both  the  power  and  resistance,  and  nearest  the  re 
sistance.  Such  a  lever  is  shown  in  Fig.  16.  The  power  is 
applied  at  7?,  the  resistance  at  A,  and  the  fulcrum  is  at  C. 


16. 


Lever  of  the  third  class.— In  this  class  the  fulcrum  is  be 
yond  both  the  power  and  the  resistance,  and  nearest  the 
power,  as  shown  in  Fig.  17. 

In  every  class  of  lever,  the  distances  from  the  fulcrum,  to 
the  power  and  resistance,  are  called  Lever  Arms.  In  each 
of  the  figures  in  this  article,  CB  is  the  lever  arm  of  the 
power,  and  CA  the  lever  arm  of  the  resistance. 


Conditions   of  Equilibrium  of  the   Lever. 

31.  It  is  demonstrated  in  Mechanics  (Art.  78),  that  the 
effect  of  a  force  produced  by  the  aid  of  a  lever  increases  as 
its  lever  arm  increases,  so  that,  if  the  lever  arm  be  doubled 
or  tripled,  the  effect  of  the  force  is  always  doubled  or  tripled. 


What  are  the  Lover  Arms  ?    (31.)  What  is  the  relation  between  the  power  and 
resistance  ? 


36 


TOrULAK     PHYSICS. 


Hence  it  was  that  ARCHIMEDES  was  able  to  say,  that  he 
could  lift  the  world  if  he  had  a  place  on  which  to  rest  his 
lever. 


Fig.  17. 


Since  the  effect  of  a  force  increases  with  its  arm  of  lever 
it  is  necessary,  in  order  that  the  power  and  resistance  may  be 
in  equilibrium,  that  they  should  be  to  each  other  inversely  as 
their  lever  arms.  That  is,  if  the  power  is  three  times  the 
resistance,  the  lever  arm  of  the  former  should  only  be  one 
third  as  long  as  that  of  the  latter,  and  so  on.  If  the  power 
is  equal  to  the  resistance,  they  will  be  in  equilibrium  when 
their  lever  arms  are  equal. 

From  what  has  been  said,  it  follows,  that  the  power  is  always 
greater  than  the  resistance  in  the  third  class  of  levers,  and  less  than 
it,  in  the  second  class.  In  the  first  class  the  power  may  be  either 
greater  or  less  than  the  resistance.  We  say  in  common  language 

Between  the  power  and  velocity? 


MECHANICAL   PUIXCIPLES.  37 

that  there  is  a  loss  of  power  in  using  a  lever  of  the  third  class,  and 
a  gain  of  power  in  using  one  of  the  second  class. 

In  performing  any  work  with  a  lever,  the  paths  passed  over  by  the 
points  of  application  of  the  power  and  resistance  are  proportional  to 
their  lever  arms;  that  is,  the  longer  the  lever  arm  the  greater  the 
path  passed  over,  and  the  greater  its  velocity.  This  is  expressed  by 
saying,  that  what  is  gained  in  power  is  lost  in  velocity.  It  is  for  this 
reason  that  we  say  there  is  no  real  gain  of  power  in  the  employ 
ment  of  a  lever. 

Examples   of  Levers. 

32.  Levers  are  of  continual  use  in  the  arts,  forming 
component  parts  of  nearly  every  machine. 


Fig.  18. 

A  pair  of  scissors  affords  an  example  of  the  first  class  of 
levers.  The  fulcrum  is  at  6Y,  Fig.  18,  the  hand  furnishes 
the  power,  and  the  substance  to  be  cut  the  resistance. 

The  common  balance,  yet  to  bo  described,  is  a  lever  of  this  class 
as  is  also  the  handle  of  a  pump, 

The  ordinary  nut-cracker  is  an  example  of  levers  of  the 


Fig.  19. 


second  class.     The  fulcrum  is  at  (7,  Fig.  19;  the  power  is 
the  hand,  and  the  resistance  is  the  nut  to  be  cracked. 


Is  there  any  gain  of  power  in  using  a  lever  f     (  32  )  Applications.    Explain 
the  scissors.    The  nut-cracker. 


38 


POPULAR     PHYSICS. 


The  oars  of  a  boat  are  levers  of  the  second  class.  The  end  of  the 
oar  in  the  water  is  the  fulcrum,  the  hand  is  the  power,  and  the  boat, 
or  rather  the  resistance-  of  the  water  which  it  has  to  overcome,  is  the 
resistance.  The  shears  employed  for  cutting  metals  belong  to  this 
class  of  levers. 

The  treadle  of  a  flax-spinner,  or  of  a  lathe,  is  an  example 
of  a  lever  of  the  third  kind.  The  fulcrum  is  at  (7,  Fig.  20, 
the  foot  is  the  power,  and  the  work  to  be  done  is  the 

resistance. 


Fig.  20. 

The  bones  of  the  animal  frame  are  many  of  them  levers  of  this 
class.  Thus,  in  the  bone  of  the  forearm  in  man.  the  elbow  joint  is 
the  fulcrum,  the  muscle  attached  jnst  below  the  joint  is  the  power, 
and  a  weight  to  be  raised  is  the  resistance. 

Other  Machines. 

33.  Besides  the  lever  there  are  two  other  simple  ma 
chines,  the  cord  and  the  inclined  plane.  The  former  re- 


Oars  of  a  l>oat.    Treadle  of  a  spinner,    Hone  of  the  forearm,    (33.)  What  are  the 
other  simple  machines  ? 


GRAVITATION.  39 

quires  no  description,  and  the  latter  will  be  explained  further 
on.  From  these  machines,  as  elements,  are  formed  by  com 
bination,  the  pulley,  the  wheel  and  axle,  the  screw^  and  the 
wedge.  These  seven  make  up  what  are  commonly  called 
the  Mechanical  Power  s,  and  from  them  may  be  con 
structed  every  machine,  however  complicated.  For  a  more 
detailed  account  of  the  general  principles  of  Mechanism  and 
Machines,  the  reader  is  referred  to  Chapter  XI. 


III.- -PRINCIPLES    DEPENDENT    ON    THE    ATTRACTION    OF    GRAVITATION. 

Universal   Gravitation. 

34.  THE  earth  exerts  a  force  of  attraction  upon  all 
bodies  near  it,  tending  to  draw  them  towards  its  centre. 
This  force,  called  the  Force  of  Gravity,  when  unresisted 
imparts  motion,  and  the  body  is  said  to  fall ;  when  resisted 
it  gives  rise  to  pressure,  which  is  called  Weight. 

NEWTON  showed  that  the  force  of  gravity,  as  exhibited 
at  the  earth's  surface,  is  only  a  particular  case  of  a  general 
attraction  extending  throughout  the  Universe,  and  contin 
ually  tending  to  draw  bodies  together.  This  general  at 
traction  he  called  Universal  G-ravitation.  It  is  mutually 
exerted  between  any  two  bodies  whatever,  and  it  is  by 
virtue  of  it  that  the  heavenly  bodies  are  retained  in  their 
orbit?. 

The  law  of  universal  gravitation  may  be  easily  explained.  If  we 
take  the  mutual  attraction  of  two  units  of  mass,  at  a  unit's  distance 
from  each  other,  as  1,  then  will  their  mutual  attraction  at  any  other 
distance  be  equal  to  1  divided  by  the  square  of  that  distance ;  thus, 
if  the  distance  is  2.  their  attraction  will  be  J  of  what  it  was  at  the 

What  machines  are  formed  by  combinations  of  simple  machines?  Name  the  seven 
mechanical  powers.  (  34.)  What  is  the  Force  of  Gravity  ?  "What  is  its-  effect  when 
unresisted?  When  resisted?  What  is  Universal  Gravitation?  Explain  the  law 
of  Universal  Gravitation. 


40  POPULAR   PHYSICS. 

distance  1  :  if  their  distance  is  3,  their  attraction  will  be  •£•  of  what 
it  was  at  the  distance  1,  and  so  on.  If  one  of  the  masses  contains 
m  units  of  mass,  and  the  other  one  unit,  the  force  will  be  m  times  as 
great  as  though  they  were  both  units  of  mass  •  that  is,  the  attraction 
will  be  equal  to  m.  divided  by  the  square  of  the  distance  between 
the  bodies.  If  the  second  body  contain  n  units  of  mass,  the  attraction 
will  be  n  times  as  great  as  before ;  that  is,  it  will  be  mn,  divided  by 
the  square  of  the  distance  between  the  bodies. 

This  law,  discovered  by  NEWTON,  may  be  expressed  as 
follows :  Any  two  bodies  exert  upon  each  other  a  mutual 
attraction,  ichich  varies  directly  as  the  product  of  their 
masses,  and  inversely  as  the  square  of  their  distance  apart. 

Effect   of  Gravitation  on  the   Planets. 

85.  It  is  by  the  influence  of  gravitation  that  the  planets 
are  retained  in  their  orbits.  Their  motion  is  the  same  as 
though  they  had  been  projected  into  space  with  an  impulse, 
and  then  continually  drawn  from  the  right  lines  along  which 
inertia  tends  to  carry  them,  by  the  attraction  of  the  sun. 
The  planets  also  attract  the  sun,  but  their  masses  being  ex 
ceedingly  small  in  comparison  with  that  of  the  sun,  their 
effects  in  disturbing  its  position  are  exceedingly  small.  The 
orbits  of  the  planets  are  ellipses  differing  but  little  from 
circles. 

Force   of  Gravity. 

36.  The  FORCE  OF  GRAVITY  is  that  force  of  attraction 
which  the  earth  "exerts  upon  all  bodies,  tending  to  draw 
them  towards  its  centre. 

As  has  been  stated,  it  is  only  a  particular  case  of  Uni 
versal  Gravitation.  It  is,  therefore,  subject  to  the  same 
law,  that  is,  it  varies  directly  as  the  mass  of  the  body  acted 

Enunciate  NEWTON'S  law.  (35.)  What  is  the  effect  of  gravitation  on  the  planets  ? 
What  are  the  orbits  of  planets?  (  36.)  What  is  the  Force  of  Gravity  ?  How  doea 
it  vary? 


GKAVITATION.  41 

upon,  and  inversely  as  the  square  of  its  distance  from  the 
centre-  of  the  earth. 

The  shape  of  the  earth  has  been  shown  by  careful  measurement 
to  be  that  of  a  spheroid  ;  that  is,  of  a  sphere  slightly  flattened  at  the 
poles.  The  mean  radius  is  a  little  less  than  4000  miles.  On  ac 
count  of  the  flattening  of  the  earth  at  the  poles,  different  points  are 
at  slightly  different  distances  from  the  centre,  and  consequently  the 
force  of  gravity  varies  slightly  at  different  places  on  the  surface. 
For  ordinary  purposes,  however,  we  may  regard  the  earth  as  a  per 
fect  sphere,  and  the  force  of  gravity  as  constant  all  over  its  surface. 

Vertical   and   Horizontal  Lines. 

31?.  A  VERTICAL  LINE  is  a  line  along  which  a  body 
falls  freely.  All  vertical  lines  are  directed  towards  the 
centre  of  the  earth,  but  for  places  near  together  they  may 
be  regarded  as  parallel. 

In  Fig.  21,  the  lines  ao  and  bo  are  verticals,  but  if  they  are  not 
far  apart,  their  convergence  is  so  small  that  they  may  be  taken  as 


Fig.  21. 

parallel.     If,  however,  their  distance  apart  is  considerable,  they  can 
not  be  regarded  as  parallel.     A  man  standing  erect  has  his  body  in 

What  is  the  shape  of  the  earth?  (37.)  What  is  a  Vertical  Line  ?  Where  do 
Terticals  meet  ?  When  may  they  be  considered  parallel  ?  When  not  parallel  t  Il 
lustrate. 


4:2  POPULAK   PHYSICS. 

a  vertical,  and  it  may  happen  that  two  persons  on  opposite  sides  of 
the  globe,  as  at  E  and  £',  may  both  stand  erect,  and  yet  their  heads 
be  turned  in  exactly  opposite  directions,  their  feet  being  turned 
towards  each  other.  Points  where  this  may  happen  are  said  to  be 
antipoles. 

A  HORIZONTAL  LINE,  or  PLANE,  at  any  place  is  one 
which  is  perpendicular  to  a  vertical  line  at  that  place. 
The  surface  of  still  water  is  horizontal,  or  level.  For  small 
areas  this  surface  may  be  regarded  as  a  plane,  but  when  a 
large  surface  is  considered,  as  the  ocean,  it  must  be  con 
sidered  as  curved,  conforming  to  the  general  outline  of  the 
earth's  surface. 

Upon  the  principle  of  verticals  and  horizontals,  all  of  our 
instruments  for  levelling  and  making  astronomical  observa 
tions  are  constructed. 

The  Plumb-Line. 

38.  A   PLUMB-LINE,    is  a   line   having   a  heavy  body, 
usually  of  lead,  suspended  at  one  of  its  ends.     When  the 
other  end  is  held  in  the  hand,  the  lead,  tending  towards  the 
centre  of  the  earth,  stretches  the  string  in  the  direction  of 
the  force  of  gravity. 

It  is  used  for  indicating  a  vertical  line.  In  Engineering  and 
Architecture  it  is  of  continual  use.  For  determining  whether  a  wall 
is  vertical,  it  .is  accompanied  by  a  square  plate,  whose  length  is 
just  equal  to  the  diameter  of  the  cylindrical  leaden  weight,  and 
wrhich  has  a  hole  at  its  middle  point,  just  large  enough  to  admit 
the  passage  of  the  string.  The  edge  of  the  plate  is  applied  to  the 
masonry,  as  shown  in  Fig.  22',  and  if  the  plumb-bob  just  touches  the 
wall,  it  must  be  vertical. 

Weight. 

39.  The  WEIGHT  of  a  body  is  the  pressure  which  it 


~\Yhal  are  antipodes?  What  is  a  Horizontal  Line,  or  Plane  ?  Level?  Applica 
tions  to  instruments.  (38.)  What  is 'a  Plumb-Line?  Describe  it  and  its  use. 
I  39.)  What  is  Weight? 


GRAVITATION. 


43 


exerts  upon  any  body  that  prevents  it  from  falling  towards 
the  earth. 


Fiff.  22. 


The  weight  of  a  body  is  due  to  the  force  of  gravity,  acting  upon 
all  of  its  particles,  but  it  must  not  be  confounded  with  the  force  of 
gravity.  Weight  is  only  the  effect  of  gravity  when  resisted  •  when 
gravity  is  unresisted  it  produces  quite  another  effect,  that  is,  motion. 

At  the  same  place  the  weights  of  bodies  are  proportional  to  their 
masses,  or  the  quantities  of  matter  which  they  contain  We  shall 
see  hereafter  that  the  weight  of  bodies  may  be  determined  by  means 
of  the  balance  ;  the  force  of  gravity  is  determined  by  the  velocity 
which  it  can  impart  to  a  body  in  a  certain  time,  as  will  be  shown 
more  fully  hereafter. 

Centre  of  Gravity. 

4O.  The  CENTRE  OF  GRAVITY  of  a  body  is  that  point 
through  which  the  direction  of  its  weight  always  passes. 

We*  have  seen  that  the  weight  of  a  body  is  the  resultant 
of  the  action  of  gravity  upon  all  of  its  particles.  It  is  shown 


Is  weight  the  (tame  as  gravity  ?    ITmo  is  weight  determined  f 
measured  ?    (  4O.)  What  is  the  Centre  of  Gravity  ? 


How  is  gravity 


POPULAR   PHYSICS. 


in  Mechanics,  that  whatever  may  be  the  form  of  a  body,  or 
whatever  may  be  its  position,  the  direction  of  its  weight 
always  passes  through  a  single  point.  This  point  is  the 
centre  of  gravity. 

The  determination  of  the  centre  of  gravity  in  the  general  case 
requires  the  aid  of  mathematics,  but  in  many  cases  its  position  is 
evident.  In  a  uniform  straight  bar,  it  is  at  the  middle  point.  In  a 
square,  or  a  rectangular,  or  a  circular,  or  an  elliptical  disk  it  is  at  the 
centre,  or  middle  point. 

Equilibrium  of  heavy  Bodies. 

41.  The  centre  of  gravity  being  the  point  at  which  the 
weight  is  applied,  it  follows  that,  if  this  point  is  held  fast  by 
any  support  whatever,  the  effect  of  the  weight  is  completely 
counteracted,  and  the  body  will  be  in  a  state  of  equilibrium. 

If  a  body  has  but  a 
single  point  of  support, 
it  can  be  in  equilibrium 
only  when  its  centre  of 
gravity  lies  somewhere 
on  a  vertical  through  that 
point.  An  example  is 
shown  in  Fig.  23,  which 
represents  a  boy  balanc 
ing  a  cane  upon  his  finger. 
In  the  figure,  g  is  the 
centre  of  gravity,  and 
that  point  must  be  kept 
exactly  over  the  point  of 
support.  This  is  a  case 
of  unstable  equilibrium. 

If  a  body  has  but  two 
points  of  support,  it  can 


v'-v  •-;  -• 

Fig.  23. 


Where  in  the  centre  of  gravity  of  a  straight  line?  Of  a  square  f  Rectangle  ? 
Circle?  Ellipse?  (41.)  When  is  a  body  in  equilibrium?  When  a  body  rests  on 
a  point,  where  must  the  centre  of  gravity  be  ?  Example. 


GRAVITATION.  45 

be  in  equilibrium  only  when  its  centre  of  gravity  lies  in  a 
vertical  drawn  through  some  point  of  the  line  joining  these 
two  points,  An  example  is  shown  in  Fig.  24,  which  repre 
sents  a  man  standing  on  stilts.  To  be  in  equilibrium,  his 
centre  of  gravity  must  be  exactly  over  the  line  joining  the 
feet  of  his  stilts.  This  is  also  a  case  of  unstable  equi 
librium. 


Fig.  24. 


Fig.  25. 


The  art  of  balancing,  in  which  circus-riders  and  rope-dancers  are 
so  expert,  consists  in  skillfully  keeping  the  centre  of  gravity  sup 
ported. 

If  a  body  has  three  supports  not  in  a  straight  line,  it  will 
be  in  equilibrium  when  the  centre  of  gravity  lies  on  a  ver 
tical  drawn  through  any  point  of  the  triangle  formed  by 
joining  these  points.  An  example  is  shown  in  Fig.  25, 
which  represents  a  three-legged  table.  The  centre  of 
gravity  being  at  </,  the  table  will  be  in  equilibrium  so  long 
as  the  vertical  through  that  pouat  pierces  the  triangle 
formed  by  uniting  the  feet  of  the  table. 


When  it  rests  on  two  points  ?    Example.    When  on  three  points  ?    Example 


46  POPULAR     PHYSICS. 

If  a  body  has  four  or  more  supports,  the  condition  ot 
equilibrium  will  be  analogous  to  that  just  explained.  In 
this  case,  if  the  outer  points  of  support  be  joined  by  lines, 
they  will  form  a  polygon,  called  the  Polygon  of  Support, 
and  the  body  will  be  in  equilibrium  when  its  centre  of 
gravity  is  on  a  vertical  drawn  through  any  point  of  this 
polygon. 

Different  kinds   of  Equilibrium. 

42.  When  bodies  are  acted  upon  only  by  the  force  of 
gravity,  and  have  one  or  more  points  of  support,  three  kinds 
of  equilibrium  may  exist :  Stable,  Unstable,  and  Neutral 
Equilibrium. 

1.  Stable  Equilibrium. — A  body  is  in  stable  equilibrium, 
when,  on  being  slightly  disturbed  from  its  state  of  rest,  it 
tends  of  itself  to  return  to  that  state. 


Fig.  26.  Fig.  27. 

This  will  be  the  case  when  the  centre  of  gravity  is  lower 
in  its  position  of  rest  than  it  is  in  any  of  the  neighboring 
positions,  for  in  this  case  the  weight  of  the  body  acting  at 


When  on  four  or  more  points?    What  is  the  Polygon  of  Support?    (  42.)  What 
are  the  three  cases  of  equilibrium  ?    What  is  Stable  Equilibrium  ?    Illustrate. 


GRAVITATION.  47 

the  centre  of  gravity  tends  to  keep  it  in  the  lowest  position. 
If  slightly  disturbed  from  the  lowest  position,  the  weight 
will  act  to  draw  it  back,  and  so  establish  the  equilibrium. 

We  have  an  example  of  stable  equilibrium  represented  in  Figs.  26 
and  27.  which  represent  images  often  met  with  in  the  toy  shops. 
If  the  image  be  inclined  to  one  side,  as  shown  in  Fig.  27,  it  will  by 
its  own  weight  right  itself,  and  take  the  position  shown  in  Fig.  26. 
These  figures  are  hollow  and  light,  and  are  ballasted  with  lead  at 
their  lower  part  so  as  to  throw  the  centre  of  gravity  very  low.  The 
result  is,  that  when  the  figure  is  inclined,  the  centre  of  gravity  is 
raised,  and  the  weight  acts  to  restore  it.  The  figure  settles  in  its  prim 
itive  state  of  rest  only  after  several  oscillations,  which  are  due  to  the 
inertia  of  the  body.  The  explanation  of  this  oscillation  is  the  same 
as  that  given  for  the  oscillation  of  the  pendulum. 

2.  Unstable  Equilibrium. — A  body  is  in  unstable  equi 
librium,  when,  on  being  slightly  disturbed  from  its  state  of 
rest,  it  does  not  tend  to  return  to  that  state,  but  continues 
to  depart  from  it  more  and  more. 

This  will  be  the  case  when  the  centre  of  gravity  is  higher 
in  its  position  of  rest  than  in  any  of  the  neighboring  posi 
tions.  When  the  body  is  slightly  disturbed,  the  weight 
acts  not  only  to  prevent  its  return,  but  also  to  cause  it  to 
descend  still  lower. 

We  have  examples  of  unstable  equilibrium  shown  in  Figs.  23 
and  24.  In  Fig  23.  the  cane  may  overturn  in  any  direction,  whilst 
in  Fig.  24  the  man  will  overturn  about  the  line  joining  the  bottom 
of  his  stilts. 

3.  Neutral  Equilibrium.  —  A  body  is  in  neutral  equi 
librium,  when,  on  being  slightly  disturbed,  it  has  no  tend 
ency  either  to  return  to  its  primitive  state,  or  to  depart 
further  from  it. 

This  Avill  be  the  case  when  the  centre  of  gravity  is  at  the 


Example.     What  is   Unstable  Equilibrium?     Illustrate.     Examples.    What 
Neutral  Equilibrium  ?    Illustrate. 


POPULAR     PHYSICS. 


same  height  in  its  position  of  rest  as  in  the  neighboring 
positions. 

We  have  an  example  of  this  kind  of  equilibrium  in  a  ball  resting 
on  a  horizontal  table. 


Fig.  28. 

Stability  of  Bodies. 

43.  From  what  has  been  said  in  the  preceding  articles, 
it  follows  that  bodies  will  in  general  be  most  stable  when 
their  bases  are  largest.  For  in  such  cases,  even  after  a  con 
siderable  inclination,  the  line  of  direction  of  the  weight  will 


Example.    (  43.)  What  bodies  are  most  stable  ? 


GRAVITATION.  49 

pass  within  the  original  polygon  of  support,  and  the  weight 
will  act  to  return  the  body  to  its  original  state  of  rest. 
Hence  it  is  that  we  find  chairs,  lamps,  candlesticks,  and 
many  other  familiar  utensils  constructed  with  broad  bases, 
to  render  them  more  stable. 

The  leaning  tower  of  Pisa  is  so  much  inclined  that  it  appears 
about  to  fall ;  yet  it  stands,  because  the  vertical  through  the  centre 
of  gravity  passes  within  the  base  of  the  tower.  Fig.  28  represents 
a  tower  at  Bologna,  which  is  even  more  inclined  than  that  at  Pisa. 
This  tower  was  built  in  the  year  1112.  and  received  its  inclination 
from  unequal  settling  of  the  ground  on  which  it  was  built.  It  does 
not  fall,  because  the  vertical  through  the  centre  of  gravity,  G:  passes 
within  its  base. 

In  the  cases  considered,  the  position  of  the  centre  of  gravity  re 
mains  the  same  for  the  same  body.  With  men  and  animals  the 
position  of  the  centre  of  gravity  changes  with  every  change  of  atti 
tude,  which  requires  a  proper  adjustment  of  the  feet;  to  maintain  a 
position  of  stability. 


Fig.  29.  Fig.  00. 


,     When  a  man  carries  a  burden,  as  shown  in  Fig.  29;  he  leans  for 
ward,  that  the  direction  of  his  own  weight  with   that  of  his  burden 

Explain  the  stability  of  the  towers  of  Pisa  and  Bologna.    How  do  men  and 
animals  maintain  a  stable  position  f    Illustrate. 

3 


50  POPULAR     PHYSICS. 

may  pass  between  his  feet.  When  a  man  carries  a  weight  in  one 
hand,  as  shown  in  Fig.  30,  he  throws  his  body  toward  the  opposite 
side  for  the  same  reason. 

In  the  art  of  rope-dancing,  the  great  difficulty  consists  in  keeping 
the  centre  of  gravity  exactly  over  the  rope.  To  attain  this  result 
the  more  easily,  a  rope-dancer  carries  a  long  pole,  called  a  balancing 
pole,  and  when  he  feels  himself  inclining  towards  one  side,  he  ad 
vances  his  pole  towards  the  other  side,  so  as  to  bring  the  common 
/centre  of  gravity  over  the  rope,  thus  preserving  his  equilibrium 
The  rope-dancer  is  in  a  continual  state  of  unstable  equilibrium. 

The  Balance. 

44.     A  BALANCE  is  a  machine  for  weighing  bodies. 

Balances  are  of  continual  use  in  commerce  and  the  arts, 
in  the  laboratory,  and  in  physical  researches;  they  are  con 
sequently  extremely  various  in  their  forms  and  modes  of 
construction.  We  shall  only  describe  that  form  which  is  in 
most  common  use  in  the  shops. 

It  consists  of  a  metallic  bar,  AB  (Fig.  31),  called  the 
JBeam^  which  is  simply  a  lever  of  the  first  order.  At  its 
middle  point  is  a  knife-edged  axis,  n,  called  the  Fulcrum. 
The  fulcrum  projects  from  the  sides  of  the  beam,  and  rests 
on  two  supports  at  the  top  of  a  firm  and  inflexible  standard. 
The  knife-edged  axis,  and  the  supports  on  which  it  rests,  are 
both  of  hardened  steel,  and  nicely  polished,  in  order  to  make 
the  friction  as  small  as  possible.  At  the  extremities  of  the 
beam  are  suspended  two  plates  or  basins,  called  Scale  Pans, 
in  one  of  which  is  placed  the  body  to  be  weighed,  and  in 
the  other  the  weights  of  iron  or  brass  to  counterpoise  it. 
Finally,  a  needle  projecting  from  the  beam,  and  playing  in 
front  of  a  graduated  scale,  a,  serves  to  show  when  the  beam 
is  exactly  horizontal. 

Explain  the  principle  of  rope-dancing.  (  44.)  What  is  a  balance  ?  Explain  the 
details  of  the  common  Balance.  The  Beam.  The  Fulcrum.  The  Scale  Pans.  The 
Scale. 


GRAVITATION. 


51 


To  weigh  a  body,  we  place  it  in  one  of  the  scale  pans, 
and  then  put  weights  into  the  other  pan  until  the  beam 


Fig.  81. 


becomes  horizontal.     The  weights  put  in  the   second  pan 
indicate  the  weight  of  the  body. 


How  are  bodies  weighed? 


52  POPULAR   PHYSICS. 


Requisites  for  a  good  Balance. 

45.  A  good  balance  ought  to  satisfy  the  following  con 
ditions  : 

1.  The  lever  arms,  An  and  Bn,  should  be  exactly  equal. 

We  have  seen  in  discussing  the  lever,  that  its  arms  must  be  equal, 
in  order  that  there  may  be  an  equilibrium  between  the  power  and 
resistance,  when  these  are  equal.  If  the  arms  are  not  equal,  the 
weights  placed  in  one  scale  pan  will  not  indicate  the  exact  weight 
of  the  body  placed  in  the  other. 

2.  The  balance  should  be  sensitive  /  that  is,  it  should  turn 
on  a  very  small  difference  of  weights  in  the  two  scale  pans. 

This  requires  the  fulcrum  and  its  supports  to  be  very  hard  and 
smooth,  so  as  to  produce  little  friction.  By  making  the  needle  long, 
a  slight  variation  from  the  horizontal  will  be  more  readily  per 
ceived. 

3.  The  centre   of  gravity  of  the  beam  and   scale  pans 
should  be  slightly  below  the  edge  of  the  fulcrum. 

If  it  were  in  the  edge  of  the  fulcrum,  the  beam  would  not  come 
to  a  horizontal  position  when  the  scales  were  equally  loaded,  but 
Would  remain  in  any  position  where  it  might  chance  to  be  placed. 
If  it  were  above  the  edge  of  the  fulcrum,  the  beam  would  remain 
horizontal  if  placed  so,  but  if  slightly  deflected,  it  would  tend  to 
overturn  by  the  action  of  the  weight  of  the  beam. 

The  nearer  the  centre  of  gravity  comes  to  the  edge  of 
the  fulcrum,  the  more  accurate  it  will  be  ;  but  at  the  same 
time,  it  would  turn  more  slowly,  and  might  finally  come  to 
turn  too  slowly  to  be  of  use  for  weighing. 

It  is  to  be  observed  that  when  the  scale  pans  are  heavily 
loaded,  an  increased  weight  is  thrown  on  the  fulcrum,  which 


(  45.)   Explain   the  requisites  of  a  good  balance.    1.   Lever  arms.     Illustrate. 
2.  Sensitiveness.    Illustrate.    3.  Position  of  centre  of  gravity.    Illustrate. 


GRAVITATION.  53 

causes  an  increase  of  friction,  and  consequently  a  diminu 
tion  of  sensitiveness. 


Methods   of  Testing  a   Balance. 

46.  To  see  whether  the  arms  are  of  equal  length,  let  a 
body  be  placed  in  one  scale  pan,  and  counterbalanced  by 
weights  put  in  the  other ;  then  change  places  with  the  body 
and  the  weights.     If  the  beam  remains  horizontal  after  this 
change,  the  arms  are  of  equal  length,  otherwise  the  balance 
is  false. 

To  test  the  sensitiveness,  load  the  balance  and  bring  the 
beam  to  a  horizontal  position,  then  deflect  it  slightly  by  a 
small  force  and  see  whether  it  returns  sloAvly  to  its  former 
position.  It  ought  to  come  to  a  state  of  rest  by  a  succes 
sion  of  oscillations. 

Method  of  weighing   correctly  with  a  false  Balance. 

47.  To  weigh  a  body  with  a  false  balance,  place  it  in 
one  scale  pan  and  counterbalance  it  by  any  heavy  matter, 
as  shot  or  sand,  placed  in  the  other  pan.     Then  take  out 
the  body  and  replace  it  by  weights  which  will  exactly  re 
store  the  equilibrium  of  the  balance.     The  weights  will  be 
exactly  equal  to  the  weight  of  the  body.     The  reason  for 
this  method  is  apparent. 

Laws  of  falling  bodies. 

48.  When  bodies  starting  from  a  state  of  rest  fall  freely 
in  vacuum,  that  is,  without   experiencing   any  resistance, 
they  conform  to  the  following  laws : 

1.  All  bodies  fall  equally  fast. 


(  46.)  How  is  a  balance  to  be  tested  ?    (  47.)  How  may  a  body  be  weighed  cor 
rectly  by  a  false  balance  ?    (48.)  What  is  the  first  law  of  falling  bodies  ? 


POPULAR   PHYSICS. 


When  resisted  by  the  air,  bodies  whose  bulk  is  very  large  in  pro 
portion  to  their  weight,  fall  more  slowly  than  those  whose  bulk  is 
small ;  thus,  a  soap-bubble  falls  more  slowly  than  a  bullet.' 

2.  The  velocities  acquired  during  the  fall  are  propor 
tional  to  the  times  occupied  in  falling. 

A  body  acquires  a  velocity  of  32£  feet  in  one  second;  it  will 
therefore  acquire  a  velocity  of  64^-  feet  in  two  seconds,  a  velocity  of 
96^  feet  in  three  seconds,  and  so  on. 

3.  The  spaces  passed  over  are  proportional  to  the  squares 
of  the  times  occupied  in  falling. 

A  body  falls  from  rest  through 
feet  in  one  second;  it  will 
therefore  fall  4  x  1 6 T^ .  or  64J,  in  two 
seconds,  9xl6T^,  or  144f  feet,  in 
three  seconds.  16x16^.  or  257^ 
feet  in  four  seconds,  and  so  on. 

The  first  law  is  verified  by 
the  following  experiment.  A 
glass  tube,  six  feet  long  (Fig. 
32),  is  closed  at  one  end,  and 
at  the  other  it  has  a  stop-cock, 
by  which  it  can  be  closed  or 
opened  at  pleasure.  A  small 
leaden  ball  and  a  feather  are  in 
troduced  within  the  tube.  So 
long  as  the  tube  is  full  of  air,  if 
it  be  suddenly  inverted,  it  will 
be  observed  that  the  ball  reach 
es  the  bottom  sooner  than  the 
feather.  If  now  the  air  be  ex 
hausted  by  means  of  an  air- 


Effect  of  atmospheric  resistance.    What  is  the  second  law?    Illustrate     Third 
iaw?    Illustrate.    How  is  the  first  law  verified? 


GRAVITATION.  55 

pump,  and  the  tube  suddenly  inverted,  both  the  ball  and 
the  feather  will  be  seen  to  fall  through  the  length  of  the 
tube  in  the  same  time.  This  experiment,  besides  verifying 
the  law,  shows  also  that  the  air  offers  a  resistance,  which  is 
greater  for  light  than  for  heavy  bodies.  This  resistance  is 
proportional  to  the  surface  offered  to  the  direction  of  the 
Hill. 

The  second  law  is  a  consequence  of  inertia  and  the  con 
tinued  action  of  gravity.  The  velocity  generated  in  the 
first  second  is  to  be  added  to  that  generated  in  the  next 
second,  to  obtain  the  velocity  generated  in  two  seconds. 
This  must  be  twice  that  generated  in  the  first  second.  This 
again  must  be  added  to  that  generated  in  the  third  second, 
to  obtain  that  generated  in  three  seconds.  This  then  must 
be  three  times  that  generated  in  the  first  second,  and 
so  on. 

The  explanation  of  the  third  law  will  be  better  under 
stood  after  having  considered  the  nature  of  the  inclined 
plane,  which  is  discussed  in  the  succeeding  articles. 

The  Inclined  Plane. 

49.  An  INCLINED  PLANE  is  a  plane  which  is  inclined  to 
a  horizontal  plane ;  thus,  A B,  Fig.  33,  is  an  inclined  plane. 

When  a  body  rests  on  a  horizontal  plane,  as  for  example 
on  a  table,  the  action  of  gravity  tending  to  draw  it  down 
is  completely  counteracted  by  the  resistance  of  the  plane, 
and  it  remains  at  rest.  It  is  not  so,  however,  when  a  body 
is  placed  upon  an  inclined  plane.  In  this  case,  the  action  of 
gravity  may  be  resolved  into  two  components,  one  perpen 
dicular  to  the  plane,  and  the  other  parallel  to  it.  The 
action  of  the  first  component  is  counteracted  by  the  re 
sistance  of  the  plane,  whilst  the  second  component  causes 


What  other  principle  does  the  experiment  show?      Explain  the  reason  of  the 
second  law.    (  49.)  What  is  an  Inclined  Plane  ?    Explain  its  principle. 


56  POPULAR   PHYSICS. 

the  body  to  move  down  the  plane.  Now,  this  last  force  is 
only  a  fraction  of  the  weight  of  the  body,  as  a  fourth,  a 
fifth,  or  a  sixth,  according  to  the  inclination,  but  it  obeys 
the  same  laws  that  the  entire  force  would,  in  causing  a  body 
to  fill. 


Verification  of  the  third  Law  of  falling  Bodies. 

5O.  To  verify  the  third  law  of  falling  bodies,  we  con 
struct  a  plane  with  a  slight  inclination  and  divide  it  into 
100  equal  parts,  as  shown  in  Fig.  33.  We  then  ascertain 
by  successive  trials  at  what  division  of  the  scale  a  leaden 


Fig. 


ball  must  be  placed  to  roll  to  the  bottom  A,  in  one  second; 
suppose  at  the  sixth  division.  If  now  the  ball  be  placed  at 
the  twenty-fourth  division,  it  will  roll  to  the  bottom  in  two 
seconds ;  if  placed  at  the  fifty-fourth  division,  it  will  roll 
down  in  three  seconds ;  if  placed  at  the  ninety-sixth  divi 
sion,  it  will  roll  down  in  four  seconds,  and  so  on. 

Hence,  Ave  conclude  that,  the  spaces  passed  over  are  pro 
portional  to  the  squares  of  the  times. 

(  50.)  How  is  the  third  law  of  falling  bodies  verified  ? 


GRAVITATION. 


57 


Applications  of  the  Inclined  Plane. 

51.  When  a  body  is  placed  upon  an  inclined  plane,  that 
component  of  its  weight  which  acts  to  move  it  down  the 
plane,  becomes  smaller  as  its  inclination  diminishes.  Hence, 
the  force  required  to  draw  a  body  up  an  inclined  plane, 
will  become  smaller  as  the  inclination  diminishes.  This 
principle  is  often  utilized  in  the  Arts ;  thus,  to  raise  a  heavy 
body  to  a  height,  we  construct  an  inclined  plane,  up  which 
it  may  be  easily  drawn. 

It  is  in  accordance  with  this  principle  that  roads  are  con 
structed  to  ascend  high  hills  and  mountains,  as  shown  in 
Fig.  34.  Such  a  road  consists  of  a  succession  of  planes 


lying  in  diiferent  directions,  which  may  be  equally  or  un 
equally  inclined  to  the  horizon. 


(51.)  What  is  the  use  of  the  inclined  plane  In  the  Arts  ?    Explain  its  application 
to  roads. 

3* 


58 


POPULAR    PHYSICS. 


It  Is  according  to  the  principle  of  the  inclined  plane  that 
water  flows  along  rivers  and  canals.  The  steeper  the  in 
clined  planes  which  form  their  beds,  the  more  rapid  their 
currents. 

In  mechanics,  two  inclined  planes,  wound  about  a  cylin 
der,  constitute  the  screw ;  hence  the  principle  of  the  screw 
is  but  a  modification  of  that  of  the  inclined  plane.  The 
wedge  is  made  up  of  two  inclined  planes,  placed  back  to 
back ;  hence  its  principle  is  also  but  a  modification  of  that 
of  the  inclined  plane. 


The  Pendulum. 


52.  A  PENDULUM  is  a  heavy  body  suspended  from  a 
horizontal  axis  about  which  it  is  free  to  vibrate.  Thus,  the 
ball  m,  suspended  from  (7,  by  a  string,  Figs.  35  and  36,  is  a 
pendulum. 


When  the  centre  of  the  ball. 
m,  is  exactly  below  the  point 
of  suspension  C,  Fig.  35,  it  is 
in  equilibrium,  for  in  that  po 
sition  the  action  of  gravity  is 
resisted  by  the  tension  of  the 
string.  If,  however,  the  ball 
be  drawn  aside  to  n.  Fig.  36, 
it  is  no  longer  in  equilibrium, 
for  in  that  position  the  force 
of  gravity  acts  to  draw  it  back  -pig.  35. 

to  m,  at  which    point   it  will 

arrive  with  the  same  velocity  as  though  it  had  tallen  through  the 
vertical  height  om.  In  consequence  of  its  inertia  Tind  acquired 
velocity,  the  ball  does  not  stop  at  m,  but  moves  on  towards  p.  In 
descending  from  n  to  m,  the  force  of  gravity  acts  as  an  accelerating 


Fig.  86 


Explain  the  flow  of  rivers.  What  Is  the  screw?  What  is  the  wedge ?  On  what 
principle  do  they  act  ?  (52.)  What  is  a  Pendulum?  What  causes  the  pendulum 
to  vibrate  f  Explain  the  a<tion  in  detail. 


GRAVITATION.  59 

force,  but  hi  ascending  from  m  to  p,  it  acts  as  a  retarding  force, 
hence  the  ball  moves  slower  and  slower  till  it  reaches  p.  The  dis 
tance  mp  would  be  rigorously  equal  to  mn.  were  it  not  for  the  re 
sistance  of  the  air. 

The  ball,  having  reached  p.  is  in  the  same  state  as  it  was  at  n  ; 
the  weight  again  acts  to  draw  it  back  to  TTZ,  whence,  by  virtue  of  its 
inertia  and  velocity,  it  again  rises  to  n,  and  so  on  indefinitely. 

This  backward  and  forward  motion  is  called  Oscillatory  Motion. 
A  single  excursion  from  a  to  p,  or  from  p  to  n,  is  called  a  Simple 
Oscillation,  or  Vibration  An  excursion  from  n  top,  and  back  again 
to  n,  is  called  a  Double  Oscillation.  The  angle,  pCn.  is  called  the 
angle  of  the  Amplitude  of  the  oscillation 

In  consequence  of  the  resistance  of  the  air,  the  amplitude  is  con 
tinually  diminishing,  and  the  ball  eventually  comes  to  rest,  though 
often  not  till  after  the  lapse  of  some  hours. 

Simple  and  Compound  Pendulums. 

53.  A  SIMPLE  PENDULUM  is  such  a  pendulum  as  would 
be  formed  by  suspending  a  single  material  point,  by  a 
string  destitute  of  weight. 

Such   a   pendulum  may  exist  in  theory,   and  is  thus   useful  in 

arriving  at  the  laws  of  oscillation,  but  in  practice  it  can  only  be 

approximated  to  by  making  the  ball  very  small  and  the  string  very 
fine. 

A  COMPOUND  PENDULUM  is  any  heavy  body  which  is  free 
to  oscillate  about  a  horizontal  axis. 

It  may  be  of  any  form,  but  in  general  it  consists  of  a  stem  T, 
Fig.  38.  which  is  either  of  wood  or  metal.  The  stem  terminates 
above  in  a  thin  and  flexible  plate,  a.,  usually  of  steel ;  it  terminates 
below  in  a  disk  of  metal  L.  called  the  bob,  which  disk  is  of  a  len 
ticular  shape,  that  the  resistance  of  the  air  to  its  motion  may  be  as 
little  as  possible. 

What  U  Oscillary  Motion?  What  is  an  Oscillation  or  Vibration?  What  is 
its  Amplitude?  What  effect  has  the  air  on  vibration?  (53.)  What  is  a  Simple 
Pendulum  ?  Is  it  real  or  ideal  t  What  is  a  Compound  Pendulum  ?  Explain  its 
construction. 


60  POPULAR  PHYSICS. 


Laws   of  Oscillation   of  the   Pendulum. 

54.  The  oscillations  of  the  pendulum  take  place  in 
accordance  with  the  following  laws  : 

1.  For  pendulums  of  unequal  lengths,  the  times  of  oscil 
lation  are  proportional  to  the  square  roots  of  their  lengths. 

2.  For  the  same  pendulum,  the  time  of  oscillation  is  inde 
pendent  of  the  amplitude,  provided  the  amplitude  he  small. 

3.  For  pendulums  of  the  same  length,  the  time  of  oscilla 
tion  is  independent  of  the  nature  of  the  material. 

Pendulums  of  wood,  iron,  copper,  glass,  all  being  of  the  same 
length,  will  all  oscillate  in  the  same  time. 

4.  For  the  same  pendulum  at  different  places,  the  times 
of  oscillation  are  inversely  as  the  square  roots  of  the  force 
of  gravity  at  those  places. 

These  laws  are  deduced  from  a  course  of  mathematical  reasoning 
on  the  theoretical  simple  pendulum,  but  they  may  be  verified  experi 
mentally  by  employing  a  very  small  ball  of  platinum,  or  other  heavy 
metal,  and  suspending  it  with  a  very  fine  silk  thread. 

To  verify  the  first  law  with  such  a  pendulum,  we  begin  by  making 
it  vibrate,  and  then  counting  the  number  of  vibrations  in  one  minute. 
Suppose,  for  example,  that  it  makes  seventy-two  per  minute.  Now 
make  the  string  four  times  as  long  as  before,  and  it  will  be  found 
that  the  pendulum  makes  only  thirty-six  oscillations  per  minute. 
If  the  string  is  made  nine  times  as  long  as  in  the  first  instance,  it  will 
be  found  that  the  pendulum  makes  only  twenty-four  oscillations  per 
minute,  and  so  on.  In  the  second  case  the  time  of  oscillation  is  twice 
as  great,  and  in  the  third  case  it  is  three  times  as  great  as  in  the  first 
case.  Now.  because  two,  three,  &c.,  are  the  square  roots  of  four, 
nine,  &c.,  it  follows  that  the  law  is  verified. 

To  verify  the  second  law,  let  the  same  pendulum  oscillate,  at  first 


(54.)  What  is  the  first  law  of  vibration  ?  The  second  law  ?  The  third  law  ?  Illus 
trate.  The  fourth  law?  Uow  are  the«e  laws  deduced?  IIow  is  ike  first  law 
verified  t  How  is  the  second  law  verified  t 


GRAVITATION.  61 

through  an  arc.  pn,  and  then  through  any  other  arc.  rg ;  it  will  be 
found  that  the  number  of  oscillations  per  minute  is  Ihe  same  in  each 
case.  Hence  the  law  is  verified.  It  is  to  be  observed  that  the  law 
does  not  hold  true  unless  the  arcs,  pn  and  rg,  are  very  small,  that 
is,  not  more  than  three  or  four  degrees. 

The  property  of  pendulums,  that  their  times  of  oscillation  are 
independent  of  the  amplitude  of  vibration,  is  designated  by  the  name 
isochronism,  from  two  Greek  words  signifying  equal  times;  oscilla 
tions  performed  in  equal  times  are  called  isochronal. 

GALILEO  first  discovered  the  fact  that  small  oscillations  of  a  pen 
dulum  were  isochronal,  towards  the  end  of  the  sixteenth  century.  It 
is  stated  that  he  was  led  to  the  discovery  by  noticing  the  oscillations 
of  a  chandelier  suspended  from  the  ceiling  of  the  Cathedral  of  Pisa. 

Applications   of  the  Pendulum. 

55.  On  account  of  the  isochronism  of  its  vibrations,  the 
pendulum  has  been  applied  to  regulate  the  motion  of 
clocks.  It  was  first  used  for  this  purpose  in  1657,  by 
HUYGHENS,  a  Dutch  philosopher.  The  motive  power  of  a 
clock  is  sometimes  a  weight  acting  by  a  cord  wound  around 
a  drum,  and  sometimes  a  coiled  spring  similar  to  a  watch 
spring.  These  motors  act  to  set  a  train  of  wheel-work  in 
motion,  which  in  turn  imparts  motion  to  the  hands  that 
move  round  the  dial  to  point  out  the  hour.  It  is  to  impart 
uniformity  of  motion  to  this  train  of  wheel-work  that  the 
pendulum  is  used. 

Fig.  38  shows  the  mechanism  by  means  of  which  the  pendulum 
acts  as  a  regulator.  A  toothed  wheel,  7?,  called  a  scape  wheel,  is 
connected  with  the  train  driven  by  the  motor,  and  this  scape  wheel  is 
checked  by  an  anchor,  mn,  which  is  attached  to  the  pendulum  and 
vibrates  with  it.  The  anchor  has  two  projecting  points,  m  and  72, 
called  pallets,  which  engage  alternately  with  the  teeth  of  the  scape 
wheel,  in  such  a  manner  that  only  one  tooth  can  pass  at  each  swing 

Limitation.  What  is  isochronism  ?  When  are  vibrations  isochronal?  Who 
discovered  the  pendulum,  and  when?  (55.)  What  is  the  principal  use  of  the 
pendulum  ?  What  is  the  motor  in  a  clock  ?  What  is  the  use  of  the  pendulum  ? 
Explain  the  action  of  the  pendulum  as  a  regulator. 


POPULAR    PHYSICS. 


of  the  pendulum.  The  motor  turns  the  scape  wheel  in  the  direction 
of  the  arrow  until  one  of  the  teeth  comes  in  contact  with  the  pallet, 
m,  which  stops  the  motion  of  the  wheel-work 
till  a  swing  of  the  pendulum  lifts  the  pallet,  m, 
from  betwreen  the  two  teeth,  when  a  single 
tooth  passes  and  the  wheel-work  moves  on 
until  again  arrested  by  the  pallet,  n,  falling 
between  two  teeth  on  the  other  side.  A 
second  swing  of  the  pendulum  lifts  out  the 
pallet,  77,.  suffers  another  tooth  to  pass,  when 
the  wheel-work  is  again  arrested  by  the  pallet, 
m,  and  so  on  indefinitely.  The  beats  of  the 
pendulum  being  isochronous,  the  interval  of 
time  between  the  consecutive  escape  of  two 
teeth  is  always  constant,  and  thus  the  motion 
of  the  wheel- work  is  kept  uniform.  The  loss 
of  force  which  the  pendulum  continually  ex 
periences,  is  supplied  by  the  motor  through 
the  scape  wheel  and  the  anchor.  This  is 
called  the  sustaining  power  of  the  pendulum. 
Owing  to  expansion  and  contraction  from 
variations  of  temperature,  the  length  of  the 
pendulum  varies,  and  according  to  the  first 
law.  its  time  of  vibration  changes.  In  nice 
clocks  this  change  is  compensated  by  a  com 
bination  of  metals.  In  common  clocks,  it  is 
rectified  by  lengthening  or  shortening  the  pen 
dulum  by  a  nut  and  screw,  shown  at  v.  by 
means  of  which  the  lenticular  bob  may  be 
moved  up  and  down.  In  summer  the  pendu 
lum  elongates  and  the  clock  loses  time,  or  runs 
too  slow ;  this  is  rectified  by  screwing  up  the 
nut  and  shortening  the  pendulum.  In  winter 
the  pendulum  contracts  and  the  clock  gains 
time :  this  is  rectified  by  unscrewing  the  nut 
and  lengthening  the  pendulum. 


Fig.  38. 


What  effect  have  variations  of  temperature  on  the  pendulum  ?  How  are  these 
effects  compensated  in  nice  clocks  ?  How  in  common  clocks  f  Why  do  clocks  lose 
time  in  summer  and  gain  time  in  winter  f 


GRAVITATION. 


63 


In  accordance  with  the  principle  enunciated  in  the  fourth  law.  the 
pendulum  has  been  used  to  determine  the  intensity  of  gravity  at 
different  points  on  the  earth's  surface.  In  this  way  it  has  been 
shown  that  the  velocity  acquired  by  a  body  falling  in  vacuum  for 
one  second,  is  32£  feet,  in  the  latitude  of  the  city  of  New  York.  It 
has  been  found  by  careful  experiment  that  the  length  of  a  pendulum 
vibrating  seconds  in  New  York,  is  a  little  over  39  inches. 

The  length  of  the  seconds  pendulum  at  any  place  being- 
constant,  it  has  been  taken  as  the  basis  of  the  English 
system  of  weights  and  measures,  and  from  the  English  we 
have  taken  our  own  system. 

The  pendulum  has 
been  successfully  em 
ployed  by  M.  Fou- 
CAULT,  a  French  phy 
sicist  of  our  own  day, 
to  demonstrate  the 
daily  rotation  of  our 
globe.  The  details 
of  his  experiment  are 
too  abstruse  to  be 
given  in  this  place. 

The  Metronome. 

56.  The  METRO 
NOME  is  a  sort  of 
pendulum  employed 
by  musicians  and 
others  to  mark  equal 
intervals  of  time.  It 
is  shown  in  Fig.  39. 
It  consists  of  a  pen- 


Fig.  30. 


What  principle  enables  us  to  measure  the  force  of  gravity  f  How  far  does  a 
l>ody  fall  in  one  second?  What  is  the  length  oj  a  seconds  pendulum  in,  New 
York?  Application  to  weights  and  measures?  What  application  did  FOUCAULT 
make  of  the  pendulum  ?  (  56.)  What  is  a  Metronome? 


POPULAR     PHYSICS, 


dulum  CB,  suspended  at  0.  A  weight,  A,  slides  along  tho 
rod  (7,  and  may  be  set  so  as  to  make  the  vibrations  as  slow 
or  as  rapid  as  may  be  desired.  The  instrument  is  set  by 
means  of  a  scale,  marked  on  the  rod,  so  that  any  number 
of  oscillations  may  be  made  in  a  minute.  The  pendulum  is 
sustained  by  a  coiled  spring  which  sets  in  motion  a  train 
of  wheels,  somewhat  in  the  manner  of  a  clock.  In  the 
drawing  the  weight  is  set  at  92,  which  shows  that  it  is  to 
make  92  oscillations  per  minute. 


IV.  —  PRINCIPLES    DEPENDENT    ON    MOLECULAR    ACTION. 

Molecular  Forces. 

57.  BESIDES  the  forces  which  act  upon  bodies  from 
without  and  at  sensible  distances,  there  is  another  class  of 
forces  continually  exerted  between  the  molecules  of  bodies, 
and  acting  only  at  insensible  distances.  These  forces  are 
called  Molecular  Forces,  and  are  both  attractive  and 
repellent. 

The  molecules  of  bodies  are  held  in  equilibrium  by  these  forces, 
and  it  is  to  them  that  are  to  be  attributed  many  of  the  most  im 
portant  physical  properties.  The  ultimate  particles  of  bodies  do  not 
touch  each  other,  being  kept  asunder  by  a  force  of  repulsion,  which 
we  have  said  is  in  general  due  to  heat ;  they  are  prevented  from 
receding  from  each  other  too  far  by  a  force  of  attraction,  and  it  is 
only  when  these  forces  just  balance  each  other  throughout  the  body, 
that  it  is  in  equilibrium. 

When  a  body  is  compressed,  the  forces  of  repulsion  are  called  into 
play,  and,  acting  like  coiled  springs,  they  tend  to  restore  the  body 
to  its  primitive  form.  In  like  manner,  when  a  body  is  elongated,  or 
stretched,  the  forces  of  attraction  are  called  into  action  and  tend  to 
restore  the  body  to  its  primitive  form. 

Describe  it.  (  57.)  What  are  Molecular  Forces?  How  divided?  How  are  mole- 
cules  held  in  place  f  To  what  is  the  repellent  force  due  f  Explain  the  effects  of 
compressing  and  stretching  bodies. 


MOLECULAR   ACTION.  65 


Cohesion. 

58.  COHESION  is  the   force   of  attraction  which   holds 
the  molecules  of  the  same  body  together,  as,  for  example, 
in  a  mass  of  iron,  or  of  wood. 

Cohesion  differs  from  chemical  affinity,  which  determines  the 
molecules  by  uniting  dissimilar  atoms  according  to  fixed  laws.  Chem 
ical  affinity  unites  atoms  of  carbon,  oxygen,  and  hydrogen,  to  form 
molecules  of  sugar ;  but  it  is  cohesion  that  unites  the  molecules  of 
sugar  into  a  solid  body. 

The  strength  of  bodies  depends  upon  cohesion.  When  a 
body  offers  a  strong  resistance  to  forces  tending  to  tear  it 
asunder,  it  is  said  to  be  tenacious ;  for  example,  iron  or 
steel  wires,  and  the  like,  are  highly  tenacious. 

Adhesion. 

59.  ADHESION  is  the  force  of  attraction  which  holds  the 
molecules  of  dissimilar  bodies  together.     Thus,  it  is  adhe 
sion  which  causes  paint  and  glue  to  adhere,  to  wood. 

If  two  polished  bodies  are  brought  into  contact,  and  pressed  to 
gether,  they  will  adhere  with  considerable  force.  If  two  plates  of 
glass  be  ground  so  as  to  fit  closely,  and  a  little  oil  be  interposed,  it 
is  very  difficult  to  separate  them.  If  two  hemispheres  of  lead  be 
pressed  together,  after  having  their  plane  surfaces  well  polished, 
they  will  adhere  very  strongly. 

It  is  adhesion  which  renders  it  difficult  to  raise  a  wooden  board 
from  the  surface  of  the  water  on  which  it  floats.  It  is  also  adhesion 
between  the  particles  of  wood  and  water,  that  causes  water  to 
spread  over  a  piece  of  wood  upon  which  it  is  poured. 

Solution  is  due  to  adhesion.  Thus,  when  sugar  dissolves  in 
water,  it  is  because  the  adhesion  between  the  molecules  of  sugar 


(58.)  What  is  Cohesion?  Example.  Difference  between  coJiesion  and  chem 
ical  affinity f  Illustrate.  When  is  a  body  tenacious  ?  (59.)  What  is  Adhesion? 
Example.  Explain  adhesion  of  metallic  surfaces.  Of  board  to  water.  Explain 
wnenon  of  solution. 


06  POPULAR   PHYSICS. 

and  water  is  stronger  than  the  cohesion  between  the  molecules  of 
sugar.  If  a  liquid  tends  to  spread  itself  over  a  solid  body,  it  is  said 
to  wet  it,  as  water  upon  glass.  If  it  gathers  in  globules,  it  does  not 
wet  it,  as  quicksilver  upon  glass. 

Capillary  Forces. 

6ti>.  CAPILLARY  FORCES  are  molecular  forces,  exerted 
between  the  particles  of  a  solid  and  those  of  a  liquid.  They 
are  called  capillary,  because  their  effect  is  mostly  ob 
served  in  capillary  tubes,  that  is,  tubes  of  the  diameter  of 
a  hair. 

The  following  are  some  of  the  phenomena  of  capillarity : 

1.  When  a  body  is  plunged  into  a  liquid  which  is  capable 
of  wetting  it,  as  when  a  glass  rod  is  plunged  into  water,  it 
is  observed  that  the  liquid  is  slightly  elevated  about  the 
body,  taking  a  concave  form,  as  shown  in  Fig.  40. 


Fig.  40.  Fig.  41.  Fig.  42. 

2.  If  a  hollow  tube  is  used  instead  of  a  rod,  the  liquid 
Trill  also  rise  in  the  tube,  as  shown  in  Fig.  41.  The  smaller 
the  bore  of  the  tube,  the  higher  will  the  liquid  rise,  and  the 
more  concave  will  be  its  upper  surface. 

(60.)  What    are  Capillary  Forces?    Why  so  called?    Explain  the  phenomenon 
observed  when  a  glass  rod  is  plunged  in  water.    When  a  tube  is  plunged  into  water. 


MOLECULAR    ACTION.  67 

3.  When  a  tube  is  plunged  into  a  liquid  which  is  not 
capable  of  wetting  it,  as  when  glass  is  plunged  into  quick 
silver,  the  liquid  is  depressed  both  on  the  outside  and  on 
the  inside,  taking  a  convex  surface,  as  shown  in  Fig.  42. 
The  smaller  the  tube,  the  greater  will  be  the  depression, 
and  the  more  convex  will  be  the  upper  surface. 

These  capillary  phenomena  are  due  to  the  resultant  action 
of  the  cohesion  of  the  liquid  and  the  adhesion  of  the  solid 
and  liquid.  When  the  former  predominates,  the  liquid  is 
depressed  in  the  tube.  When  the  latter  predominates,  the 
liquid  is  raised  in  the  tube. 

Applications   of  Capillarity. 

61.  It  is  in  consequence  of  capillary  action  that  oil  is 
raised  through  the  wicks  of  lamps,  to  supply  the  flame  with 
combustible  matter.  The  fibres  of  the  wicks  leave  between 
them  a  species  of  capillary  tubes,  through  which  the  oil 
rises. 

If  a  piece  of  sugar  have  its  lower  end  dipped  in  water, 
the  water  will  rise  through  the  capillary  interstices  of  the 
sugar  and  fill  them.  This  drives  out  the  air  and  renders 
the  sugar  more  soluble  than  when  plunged  dry  into  water, 
in  which  case  the  contained  air  resists  the  absorption  ol 
water,  and  retards  solution. 

If  a  bar  of  lead  be  bent  into  the  form  of  a  siphon,  and  the 
short  arm  be  dipped  into  a  vessel  of  mercury,  the  mercury 
will  rise  into  the  lead  by  capillary  action,  and  flowing  over 
the  edge  of  the  vessel  will  descend  along  the  longer  branch 
and  escape  from  the  lower  extremity.  In  this  way  the 
vessel  may  be  slowly  emptied  of  the  quicksilver. 

Many  fluids  may  be  drawn  over  the  edges  of  the  contain 
ing  vessels  by  a  siphon  of  candle-wicking  or  other  capillary 
substance. 

"When  a  glass  tube  is  plunged  into  mercury.  Causes  of  the  phenomena.  (61.) 
Why  does  oil  rise  in  n  wick  ?  Water  in  sugar  ?  Explain  leaden  siphon.  Explain 
siphon  of  wicking. 


68  POPULAR    PHYSICS. 


Absorption. 

62.  ABSORPTION  is  the  penetration  into  a  porous  body, 
of  any  foreign  body,  whether  solid,  liquid,  or  gaseous. 

Carbon,  in  the  form  of  charcoal,  has  a  great  capacity  for 
absorbing  gases.  If  a  burning  coal  be  introduced  into  a 
bell-glass,  filled  with  carbonic  acid,  collected  over  mercury, 
the  volume  of  the  gas  is  diminished  by  being  absorbed  by 
the  coal.  It  is  found  that  the  charcoal  absorbs  in  this  way 
thirty-five  times  its  own  volume  of  the  gas.  Charcoal  also 
absorbs  other  gases  in  even  still  greater  quantities. 

Spongy  platinum  absorbs  hydrogen  so  rapidly  as  to  heat 
the  platinum  red-hot. 

In  vegetables  and  animals  we  have  many  examples  of  ab 
sorption.  The  roots  of  plants  absorb  from  the  earth  the 
material  necessary  to  the  growth  of  the  stem  an  cP  branches. 

In  the  animal  world,  absorption  plays  an  important  part 
in  the  process  of  nutrition  and  growth.  Animal  tissues  also 
absorb  solid  substances.  For  example,  workmen  engaged 
in  handling  lead  absorb  through  the  skin  and  lungs  more  or 
less  of  this  substance,  which  often  gives  rise  to  very  serious 
diseases. 

Imbibition. 

63.  IMBIBITION  is  the  absorption  of  a  liquid  by  a  solid 
body. 

Imbibition  is  an  effect  of  capillarity,  for  the  interstices 
between  the  molecules,  by  communicating  with  each  other, 
form  a  mass  of  capillary  tubes,  into  which  the  liquid  pene 
trates  by  virtue  of  the  capillary  forces.  Such  is  the  cause 
of  wood  and  earth  absorbing  water  and  other  liquids.  If  a 
damp  substance  be  placed  in  a  dry  and  porous  vessel,  it 


(  62.)  What  is  Absorption?    Examples.    Carbon.    Spongy  platinum.     Vegetables. 
Animals.    (  63.)  What  is  Imbibition?    What  is  the  cause  of  imbibition  ?    Examples. 


MOLECULAR    ACTIOX.  69 

will  grow  drier,  whereas,  if  placed  in  a  vessel  which  has  no 
attraction  for  water,  it  will  remain  moist. 

When  vegetable  and  animal  substances  absorb  water,  they  gener 
ally  augment  in  volume.  This  fact  explains  many  phenomena  of 
daily  observation. 

If  a  large  sheet  of  paper  be  moistened,  it  increases  in  size,  and 
again  contracts  when  dried.  This  property  is  employed  by  draughts 
men  to  stretch  paper  on  boards.  The  paper  is  moistened,  and  after 
being  allowed  to  expand,  its  edges  are  glued  to  a  drawing-board ; 
on  drying  it  is  stretched,  forming  a  smooth  surface  for  drawing  upon. 
The  same  property  causes  the  paper  to  peel  from  the  walls  of  a  room 
when  exposed  to  moisture. 

When  a  workman  would  bend  a  piece  of  wood,  he  dries  one  side 
and  moistens  the  other.  The  side  which  is  dried  contracts,  and  the 
opposite  side  expands,  so  that  the  piece  is  curved.  It  is  the  absorp 
tion  of  moisture  that  causes  the  wood-work  of  houses,  furniture,  &c., 
to  swell  and  shrink  with  atmospheric  changes,  and  which  necessi 
tates  their  being  painted  and  varnished.  Paints  and  varnishes,  by 
filling  the  pores,  prevent  absorption. 

If  two  different  liquids  be  separated  by  a  membranous  partition, 
a  current  will  be  set  up  from  each  liquid  to  the  other  through  the 
membrane,  and  after  a  time  it  will  be  found  that  there  is  a  mixture 
of  both  liquids  on  each  side  of  the  partition.  These  currents  are 
generally  unequal,  so  that  there  is  an  actual  gain  of  substance  on  one 
side  and  a  corresponding  loss  on  the  other.  The  current  that  acts 
to  produce  an  increase  on  one  side  is  called  endosmose,  and  the  oppo 
site  current  is  called  exosmose.  Thus,  if  a  bladder  filled  with  strong 
syrup  be  tied  to  the  end  of  a  glass  tube,  and  the  whole  plunged  into 
a  vessel  of  water,  the  syrup  soon  becomes  diluted  by  the  flowing  in  of 
water,  and  the  mixture  rises  in  the  tube ;  at  the  same  time  a  portion 
of  the  syrup  flows  out  and  mixes  with  the  water.  The  flowing  in 
of  the  water  is  endosmose,  and  the  flowing  out  of  the  syrup  is  exos 
mose.  Similar  results  are  obtained  by  using  other  liquids.  The 
phenomena  of  endosmose  and  exosmose  enable  us  to  explain  many 
interesting  facts  in  animal  and  vegetable  physiology. 

What,  is  the  effect  of  imbibition?  On  paper?  Application.  Effect  on  wood? 
Application.  What  are  endosmose  and  exosmose  ?  Illustrate. 


TO  POPULAR     PHYSICS. 

V.—  PROPERTIES  OF  SOLIDS  DEPENDENT  ON  MOLECULAR  ACTION. 

Tenacity. 

64.  TENACITY  is  the  resistance  which  a  body  offers  to 
rupture  when  subjected  to  a  force  of  traction;  that  is,  a 
force  which  tends  to  tear  the  particles  asunder. 

The  tenacity  of  a  body  may  be  determined  in  pounds.  For  this 
purpose  it  is  wrought  into  a  cylindrical  form,  having  a  given  cross- 
section  ;  its  upper  end  is  then  made  fast,  and  a  scale-pan  is  attached 
to  the  lower  end  ;  weights  are  then  placed  in  the  pan  until  rupture 
takes  place.  These  weights  measure  the  tenacity  of  the  body, 

Metals  are  the  most  tenacious  of  bodies,  but  they  differ  greatly 
from  each  other  in  this  respect.  The  following  table  exhibits  the 
weights  required  to  break  wires  of  I£j$T  of  an  inch  in  diameter, 
formed  of  the  metals  indicated  : 


549  Ib. 
Copper  ..............  ...........   302    " 

Platinum  .......................   274    " 

Silver  .........................    187    " 

Gold  ...........................    150    " 

Lead  ...........................     27    " 

It  has  beBn  shown  by  theory  and  confirmed  by  experiment,  that 
of  two  cylinders  of  equal  length  and  containing  the  same  amount  of 
material,  one  being  solid  and  the  other  hollow,  the  latter  is  the 
stronger. 

This  latter  principle  is  also  true  of  cylinders  required  to  support 
weights  ;  the  hollow  cylinder  is  better  adapted  to  resist  a  crushing 
force  than  the  solid  one  of  the  same  weight,  and  hence  it  is  that 
columns  and  pillars  for  the  support  of  buildings  are  made  hollow. 
This  principle  also  indicates  that  the  bones  and  quills  of  birds,  the 
stems  of  grasses  and  other  plants,  being  hollow,  are  best  adapted 
to  secure  a  combination  of  lightness  and  strength. 


(64.)  What  is  Tenacity?  How  is  it  measured?  What  bodies  are  most  tena 
cious?  Examples.  What  is  the  form  of  greatest  strength?  Application  to  grasses, 
quills,  lones,  <Sbc. 


MOLECULAR    ACTION. 


Hardness. 

65.  HARDNESS  is  the  resistance  which  a  body  offers  to 
being  scratched  or  worn  by  another.     Thus,  the  diamond 
scratches  all  other  bodies,  and  is  therefore  harder  than  any 
of  them. 

After  the  diamond  come  the  sapphire,  the  ruby,  rock- 
crystal,  &c.,  each  of  which  is  scratched  by  the  preceding 
one,  but  scratches  the  succeeding  one. 

Hardness  must  not  be  confounded  with  resistance  to  shocks  or 
compression.  Glass,  diamond,  and  rock-crystal  are  much  hardei 
than  iron,  brass,  and  the  like,  and  yet  they  are  less  capable  of  re 
sisting  shocks  and  forces  of  compression  •  they  are  more  brittle. 

An  alloy  or  mixture  of  metals  is  generally  harder  than  the  sepa 
rate  metals  of  which  it  is  composed.  Thus,  gold  and  silver  are 
soft  metals,  and,  in  order  to  make  them  hard  enough  for  coins  and 
jewelry,  they  are  alloyed  with  a  small  portion  of  copper.  In  order 
to  render  block-tin  hard  enough  for  the  manufacture  of  domestic 
utensils,  it  is  alloyed  with  a  small  quantity  of  lead. 

The  property  of  hardness  is  utilized  in  the  arts.  To  polish  bodies, 
powders  of  emery,  tripoli,  &c.,  are  used,  which  are  powders  of  very 
hard  minerals.  Diamond  being  the  hardest  of  all  bodies,  it  can  be 
polished  only  by  means  of  its  own  powder.  Diamond-dust  is  the  most 
efficient  of  the  polishing  substances. 

Ductility. 

66.  DUCTILITY  is  the  property  of  being  drawn  out  into 
wires  by  forces  of  extension. 

Wax,  clay,  and  the  like,  are  so  tenacious,  that  they  can  easily  be 
flattened  by  forces  of  compression,  and  readily  wrought  between  the 
fingers.  Such  ^bodies  are  plastic.  Glass,  resins,  and  the  like  be 
come  tenacious  only  when  heated.  Glass  at  high  temperatures  is 

(65.)  What  is  Hardness?  What  body  is  hardest?  What  bodies  come  next? 
What  are  brittle  bodies f  What  is  the  effect  of  alloying  bodies?  Explain  the 
operation  of  polishing  f  How  is  the  diamond  polished  T  What  is  the  best  polish- 
ing  substance  f  (66.)  What  is  Ductility?  Give  examples  of  plastic  bodies  f 


72  POPULAR     PHYSICS. 

so  highly  ductile,  that  it  may  be  spun  into  fine  threads  and  woven 
into  fabrics.  Many  of  the  metals,  as  iron,  gold,  silver,  and  copper, 
are  ductile  at  ordinary  temperatures,  and  are  capable  of  being 
drawn  out  into  fine  wires,  by  means  of  wire-drawing  machines. 

The  following  metals  are  arranged  in  the  order  of  their  ductility  : 
platinum,  silver,  iron,  copper,  gold,  zinc,  tin,  lead. 

Malleability. 

67.  MALLEABILITY  is  the  property  of  being  flattened  or 
rolled  out  into  sheets,  by  forces  of  compression. 

This  property  often  augments  with  the  temperature  j  every  one 
knows  that  iron  is  more  easily  forged  when  hot  than  when  cold. 
Gold  is  highly  malleable  at  ordinary  temperatures.  Gold  is  reduced 
to  thin  sheets  by  being  rolled  out  into  plates  by  a  machine ;  these 
plates  are  cut  up  into  small  squares,  and  again  extended  by  ham 
mering  until  they  become  extremely  thin.  They  are  then  cut  up 
again  into  squares,  and  hammered  between  membranes,  called  gold 
beater's  skins.  By  this  process  gold  may  be  wrought  into  leaves  so 
thin,  that  it  would  take  282.000,  placed  one  upon  another,  to  make 
an  inch  in  thickness.  These  leaves  are  employed  in  gilding  metals, 
woods,  paper,  and  the  like.  Silver  and  copper  are  wrought  in  the 
same  manner  as  gold. 

The  following  metals  are  amongst  the  most  malleable  under  the 
hammer  :  gold,  silver,  platinum,  iron,  tin,  zinc,  copper,  lead. 

When  metals  are  alloyed,  they  are  generally  harder  and  less 
malleable,  as  well  as  less  ductile. 

Is  gold  ductile  f  When  f  Give,  examples  of  ductile  metals.  (67.)  What  is 
Malleability?  Effect  of 'temperature?  How  is  gold  formed  into  sheets  ?  What  is 
the  order  of  malleability  of  metals  f  Effect  of  alloying. 


CHAPTER    II. 

MECHANICS       OF       LIQUIDS. 

I.—  GENERAL     PRINCIPLES. 

Definition  of  Hydrostatics  and  Hydrodynamics. 

G§L  THE  Mechanics  of  Liquids  is  divided  into  two 
branches  :  HYDROSTATICS,  which  treats  of  the  laws  of  equi 
librium  oi  liquids,  and  HYDRODYNAMICS,  which  treats  of  the 
laws  of  motion  of  liquids. 

Properties  of  Liquids. 
69.    The  following  properties  are  common  to  all  liquids  : 

1.  The   molecules   of  liquids    are    extremely  movable, 
yielding  to  the  slightest  force. 

There  is  very  little  cohesion  between  the  molecules  of  liquids, 
whence  their  readiness  to  slide  amongst  each  other.  It  is  to  this 
principle  that  they  owe  their  fluidity. 

2.  Liquids  are  only  slightly  compressible. 

Liquids  are  so  slightly  compressible,  that  for  a  long  time  they 
were  regarded  as  absolutely  incompressible.  In  1823,  ERSTED  de 
monstrated,  by  an  apparatus  which  he  contrived,  that  liquids  are 
slightly  compressible.  He  showed  that  for  a  pressure  of  one  at 
mosphere,  that  is,  of  15  Ibs.  on  each  square  inch  of  surface,  water 
is  compressed  the  TTOTOIF^  °^  *ts  original  volume.  Slight  as  is 


(68.)  Define  Hydrostatics.     Hydrodynamics.      (69.)  What  is  the  first  property 

of  Liquids.     llln  '.tactic.    S-'cov.f!  property  ?     Tllu't'ate. 

4 


7»  POPULAR     PHYSICS. 

the  compressibility  of  water,  it  is  nevertheless  ten  times  as  com 
pressible  as  mercury. 

3.  Liquids  are  porous,  elastic,  and  impenetrable,  like 
other  bodies. 

That  liquids  are  porous,  has  already  been  shown  (Art.  9).  That 
they  are  elastic,  is  shown  by  their  recovering  their  volume  after  the 
compressing  force  is  removed.  It  is  also  shown  by  the  fact  that  they 
transmit  sound.  Their  impenetrability  is  shown  by  plun^m*  a 
solid  body  into  a  vessel  filled  with  a  liquid.  If  there  is  no  imbibi 
tion,  a  volume  of  water  will  flow  over  the  vessel  just  equal  to  that 
of  the  solid  introduced. 

Upon  these  three  properties  of  liquids  depends  their  pro 
perty  of  transmitting  pressures  in  all  directions. 

Transmission  of  Pressures.— Principle  of  Pascal. 

•70.  Let  a  bottle  be  filled 
with  water  and  corked,  as  re 
presented  in  Fig.  43.  If  the 
cork  be  pressed  inwards,  the 
pressure  will  be  transmitted  to 
the  molecules  in  contact  with 
it ;  these  molecules  will  in  their 
turn  press  upon  the  neighbor 
ing  ones,  and  so  on  until  the 
pressure  is  finally  transmitted 
to  every  point  of  the  interior 
surface  of  the  bottle. 

It  is  shown  by  experiment 
that  £he  pressure  thus  trans 
mitted  is  equal  to  that  applied 
to  the  cork ;  that  is,  the  pres 
sure  upon  each  square  inch  of  n>.  43. 


Third  property?    Illustrate.    (  70.)  What  is  the  Principle  of  Pascal? 


GENERAL   PROPERTIES    OF   LIQUIDS.  75 

the  interior  surface  of  the  vessel  is  equal  to  that  upon  a 
square  inch  of  the  cork.  The  pressure  is  everywhere  per 
pendicular  to  the  surface,  as  shown  by  the  arrow-heads. 

This  principle  is  called  the  Principle  of  Pascal,  because 
it  was  first  demonstrated  by  BLAISE  PASCAL  in  the  seven 
teenth  century.  Upon  it  depends  the  whole  theory  of  Hy 
drostatics. 

Pressure  due  to  the  Weight  of  Liquids. 

71.  If  a  cylindrical  vessel  is  filled  with  a  heavy  liquid, 
its  weight  produces  a  pressure  upon  the  walls  of  the  vessel. 
If  we  suppose  the  liquid  divided  into  horizontal  layers  of 
equal  thickness,  it  is  plain  that  the  second  layer  from  the 
top  supports  a  pressure  equal  to  the  weight  of  the  first, 
the  third  layer  supports  a  pressure  equal  to  the  weight  of 
the  second  and  first,  and  so  on  to  the  bottom.  Hence,  the 
pressure  upon  any  layer  is  proportional  to  its  depth  below 
the  tipper  surface,  and  is  equal  to  the  weight  of  the  column 
of  fluid  above  it. 

In  consequence  of  the  principle  of  PASCAL,  this  pressure  is 
transmitted  laterally,  and  acts  against  the  sides  of  the  vessel 
with  an  equal  intensity.  Hence,  every  part  of  the  surface 
is  pressed  with  a  force  equal  to  the  weight  of  a  column 
of  liquid  whose  base  is  the  surface  pressed,  and  whose 
height  is  equal  to  the  distance  from  that  surface  to  the 
upper  level  of  the  fluid. 

The  same  principle  holds,  whatever  may  be  the  form  of 
the  vessel. 


Why  so  called?  How  illustrated?  (71.)  What  is  the  measure  of  the  pressure 
on  any  horizontal  layer  of  a  liquid  ?  How  shown?  How  is  it  transmitted  ?  What 
pressure  is  exerted  on  the  surface  of  a  containing  vessel  ? 


76 


POPULAR     PHYSICS. 


Lateral  Pressures.  —  Reaction  Wheel. 

•72.  The  fact  that  liquids  exert  lateral  pressures  upon 
the  walls  of  vessels,  is  demonstrated  by  means  of  the 
reaction  wheel.  This  wheel  is  shown  in  Fig.  44  ;  it  consists 
of  a  vertical  cylindrical 
tube  (7,  turning  freely 
in  a  ring,  n,  near  its  up 
per  extremity,  and  rest 
ing  upon  a  pivot  at  its 
lower  extremity.  Just 
above  the  pivot,  the 
tube  terminates  in  a 
cubical  box,  from  the 
faces  of  which  project 
four  tubes,  having  their 
ends  curved,  as  shown 
in  the  figure.  Water 
is  supplied  from  a  cis 
tern  through  the  funnel 
D.  When  the  water  is 
admitted,  it  flows  down 
the  tube  (7,  and  escap 
ing  through  the  curved 
tubes  at  the  bottom,  the 
wheel  is  turned  in  the 
direction  indicated  by 
the  arrow-head. 


Fig.  44. 


The  reason  of  this  will 
be  plain  from  a  considera 
tion  of  the   small  figure  a6,  which  is  a  plan  of  two  of  the  tube?. 
The  weight  of  the  water  causes  a  pressure  upon  A.  which,  were 


(72.)  How  is  the  lateral  pressure  demonstrated?    Describe  the  reaction  wheel. 
Explain  its  action. 


GENERAL   PROPERTIES    OF   LIQUIDS. 


77 


a  closed,  would  be  exactly  counterbalanced  by  the  pressure  upon 
it  •  but  a  being  open,  the  pressure  upon  A  is  not  counterbalanced, 
but  acts  from  a  towards  A.  producing  rotary  motion.  The  pressures 
in  all  of  the  tubes  conspire  to  produce  rotation  in  the  same  direction. 

Pressure  upwards. 

73.  That  liquids  exert  a  pressure  upwards  is  demon 
strated  by  means  of  the  apparatus  shown  in  Fig.  45.  It 
consists  of  a  tube  of  glass, 
Avith  a  movable  disk,  a, 
ground  so  as  to  fit  the  bot 
tom  of  the  tube.  The  disk 
being  held  closely  against 
the  tube  by  a  string,  #,  the 
whole  is  plunged  into  a  ves 
sel  of  water.  In  this  state, 
the  disk,  though  heavier 
than  water,  does  not  fall  to 
the  bottom,  showing  that  it 
is  held  in  place  by  an  up 
ward  pressure.  If  water 
now  be  poured  into  the 
tube  in  a  gentle  stream, 

the  disk  will  adhere  till  the  latter  is  filled  to  the  level  of 
the  fluid  on  the  outside.  This  shows  that  the  upward 
pressure  is  equal  to  the  weight  of  a  column  of  water  whose 
base  is  that  of  the  tube,  and  whose  altitude  is  its  distance 
below  the  upper  surface  of  the  fluid. 

The  upward  pressure  of  fluids  is  called  their  Buoyant  Effort.  It 
is  in  consequence  of  their  buoyant  effort  that  fluids  sustain  lighter 
bodies  on  their  surfaces.  The  same  principle  causes  fluids  to  buoy 
up  bodies  of  all  kinds,  diminishing  the  weight  of  heavy  ones,  and 
causing  light  ones  to  float. 


Fig.  45. 


(  73.)  How  is  upward  pressure  demonstrated?     What  is  the  buoyant  Effort  f    Its 
effect  on  bodies  f 


POPULAR     PHYSICS. 


Pressure   on  the  Bottom  of  a  Vessel  independent  of  its  Shape. 

74.  The  pressure  on  the  bottom  of  a  vessel,  arising 
from  the  weight  of  a  liquid,  is  entirely  independent  of  the 
shape  of  the  vessel,  as  well  as  of  the  quantity  of  liquid 
,which  it  contains.  It  depends  only  on  the  size  of  the  sur 
face  pressed,  and  its  distance  below  the  upper  surface  of  the 
liquid. 

This  principle  may  be  demonstrated  by  means  of  an  apparatus, 
shown  in  Fig.  46.  The  apparatus  consists  of  a  tube,  o.  firmly  at 
tached  to  the  cover  of  a  glass  vessel,  P.  By  means  of  a  screw  joint, 
different  shaped  vessels,  A:  5,  C;  may  be  attached  to  the  upper  end 


Fig.  46. 

of  the  tube.  A  disk,  t,  of  ground  glass  is  held  in  contact  with  the 
lower  end  of  the  tube  by  a  string,  which  is  secured  at  its  upper 
extremity  to  an  arm  of  a  balance. 

The  vessel,  A,  is  screwed  on.  and   filled  with  water   until   the 
downward  pressure  exactly  counterpoises   a   given  weight   in  the 

(  74.)  Docs  the  pressure  on  the  bottom  of  a  vessel  depend  upon  the  shape  of  the 
vessel  ?    Ilow  shown  ?    Explain  the  experiment  in  detail. 


GENERAL    PROPERTIES    OF   LIQUIDS. 


79 


scale-pan,  M,  when  che  upper  surface  of  the  water  is  marked  by  a 
sliding  bead,   n.      The   other  vessels,   B  and  (7,    are  successively 
screwed  on,  and  filled  with  water  up  to 
the  level,  n ;  if  any  more  water  is  poured 
into  either,  the  downward  pressure  over 
comes  the  weight,  M,  and  the  water  es 
capes  into  the  vessel,  P. 

This  principle  of  pressure  on  the 
Dottom  of  vessels  is  sometimes  called 
the  Hydrostatic  Paradox.  It  is  so 
called,  because  the  same  pressure 
may  be  obtained  by  using  very  dif 
ferent  quantities  of  the  same  liquid. 

Pascal's  Experiment. 

75.  The  following  experiment  was 
made  by  PASCAL,  in  1647.  He  fitted  into 
the  upper  head  of  a  strong  cask  a  tube  of 
small  diameter  and  about  thirty-four  feet 
in  length,  as  shown  in  Fig.  47.  The 
cask  being  filled  with  water,  he  succeeded 
in  bursting  it  by  pouring  a  comparatively 
small  quantity  of  water  into  the  tube. 
In  this  case  the  pressure  exerted  laterally 
was  the  same  as  though  the  tube  had 
been  throughout  of  the  same  diameter  as 
the  cask,  or  even  greater. 


Fig.  47. 


Hydraulic  Press. 


76.  The  principle  of  equal  pressures  has  been  applied  in 
the  construction  of  a  press,  by  means  of  which  a  single  man 
may  exert  an  enormous  power.  This  press  is  shown  in  per. 
spective  in  Fig.  48,  and  in  section  in  Fig.  49,  the  letters  in 
both  figures  corresponding  to  the  same  parts. 


What  is  this  principle  of  pressure  called  ?    Why?    (75.)  Explain  Pascal 's  ex 
periment.    (76.)  What  is  the  principle  of  the  Hydraulic  Press? 


80 


POPULAR   PHYSICS. 


The  press  consists  of  two  cylinders,  A  and  B,  of  unequal  diame« 
ters.  In  the  cylinder,  J5,  is  a  solid  piston,  C,  which  rises  as  the  water 
is  forced  into  5,  and  thus  forces  up  a  platform,  K.  The  cylinder,  A, 
forms  the  barrel  of  a  pump  by  means  of  which  water  is  raised  from 
a  reservoir,  P,  and  forced  into  the  cylinder,  B.  This  pump  is  worked 
by  a  leyer,  0,  attached  to  a  solid  piston,  a.  When  the  piston,  a,  is 
raised,  a  vacuum  is  formed  behind  it?  which  is  filled  by  water  from 


Fig.  43. 

the  reservoir,  P,  which  enters  by  opening  the  valve,  S.  When  the 
piston  is  depressed,  the  valve,  S,  closes,  the  valve,  m,  is  opened,  and 
a  portion  of  the  water  is  forced  through  the  pipe,  d.  into  the  cylinder, 
B.  By  continuing  to  work  the  piston,  a,  up  and  down,  additional 
quantities  of  water  are  forced  into  the  large  cylinder. 

Describe  the  press  in  detail.    Explain  its  action. 


GENERAL   PROPERTIES    OF    LIQUIDS. 


81 


In  consequence  of  the  principle  of  equal  pressures,  the  force  applied 
to  the  piston,  a,  is  transmitted  through  the  tube,  d,  and  is  finally 
exerted  upwards  against  the  piston,  C,  its  effect  being  multiplied  by 
the  number  of  times  that  the  section  of  the  piston,  C,  is  greater  than 
that  of  the  piston,  a.  For  example,  if  the  section  of  C  is  1 50  times  as 
great  as  that  of  a,  every  pound  of  pressure  on  the  latter  will  produce 
150  Ibs.  of  pressure  on  the  former.  This  effect  is  further  multiplied 
by  means  of  the  lever,  0.  The  pressure  exerted  upon  C,  forces  up  the 
platform.  K,  with  an  energy  that  may  be  utilized  in  compressing  any 
substance  placed  between  it  and  the  top  of  the  press,  MN.  This 
upward  pressure  may  also  be  used  for  raising  heavy  weights. 


By  varying  the  relative  dimensions  of  the  parts  of  the  machine,  an 
immense  power  maybe  exerted.  In  the  arts,  presses  of  this  kind  are 
constructed  capable  of  exerting  a  force  of  more  than  a  hundred 
thousand  pounds. 

The  hydraulic  press  is  used  in  compressing  seeds  to  obtain  oils,  in 
packing  hay,  cotton,  and  other  goods  for  shipment,  in  pressing  books 
for  the  binder,  and  in  a  great  variety  of  other  operations.  The 
immense  tubular  bridge  over  the  Menai  Straits  was  raised  from  the 
level  of  the  water  to  the  top  of  the  piers  by  means  of  presses  of  this 


Illustrate  its  power  by  an  etramj 
4* 


What  are  its  us+s  f 


2t  POPULAR     PHYSICS. 

description.  The  hydraulic  press  was  also  used  in  launching  the 
Great  Eastern,  the  heaviest  movable  structure  ever  constructed  by 
man. 

II.  —  EQUILIBRIUM     OP     LIQUIDS. 

Conditions   of  Equilibrium. 

77.  A  solid  body  is  in  equilibrium  when  its  centre  of 
gravity  is  supported,  because  the  particles  of  the  body  are 
held  together  by  cohesion.     In  liquids  the  particles  do  not 
cohere,  and  unless  restrained  they  would  flow  away  and 
spread  out  indefinitely.     A  liquid  can  be  in  equilibrium  only 
when  restrained  by  a  vessel,  or  something  equivalent.     Fur 
thermore,  each  particle  must  be  equally  pressed  in  all  direc 
tions,  which  requires  that  the  free  surface  should  be  level, 
that  is,  everywhere  perpendicular  to  the  force  of  gravity. 

In  saying  that  the  free  surface  must  be  level,  we  suppose  that  the 
liquid  is  acted  upon  only  by  the  force  of  gravity,  which  is  the  ordi 
nary  case.  If,  however,  it  is  acted  upon  by  other  forces,  the  free 
surface  must,  at  every  point,  be  perpendicular  to  the  resultant  of  all 
the  forces  acting  at  that  point;  for  if  it  were  not  so,  this  resultant 
might  be  resolved  into  two  components,  one  perpendicular  to  the 
surface,  and  the  other  parallel  to  it.  The  former  would  be  resisted 
by  the  reaction  of  the  liquid,  and  the  latter,  being  uncompensated, 
would  produce  motion,  which  is  contrary  to  the  hypothesis  of 
equilibrium. 

Level   Surface. 

78.  The  surface  of  a  liquid  is  LEVEL  when  it  is  every 
where  perpendicular  to   the  direction  of  gravity.      Small 
level   surfaces    coincide    sensibly    with    horizontal    planes. 
Large  level  surfaces  are  curved  so  as  to  conform  to  the 
general  form  of  the  earth's  surface.     That  the  surface  of  the 
ocean  is  curved  is  shown  by  the  phenomena  presented  by  a 

(  77.)  Explain  the  difference  between  equilibrium  of  solids  and  liquids.  When  is 
a  liquid  in  equilibrium  ?  How  is  the  upper  surface  irfien  other  forces  than  gravity 
act?  Why?  (  78.)  What  is  a  level  surface?  Nature  of  a  small  level  surface  ?  Of 
a  larger  one  ?  Illustrate. 


EQUILIBRIUM    OF    LIQUIDS. 


83 


ship  viewed  from  the  shore,  as  exhibited  in  Fig.  50.  As  the 
vessel  recedes,  we  first  lose  sight  of  her  hull,  then  her 
lower  sails  disappear,  then  her  higher  sails,  until  at  last  the 
entire  vessel  is  lost  to  view. 


Fir.  50. 


In  defining  a  level  surface,  we  said  that  it  is  everywhere  per 
pendicular  to  the  direction  of  gravity ;  more  strictly  speaking,  it  is 
perpendicular  to  the  resultant  of  gravity  and  the  centrifugal  force  due 
to  the  earth's  rotation  on  its  axis.  Were  it  not  for  the  centrifugal 
force,  the  surface  of  the  ocean  would  be  perfectly  spherical,  but  in 
consequence  of  that  force,  it  is  ellipsoicral ;  that  is.  the  oceans  are 
elevated  about  the  equator  and  depressed  about  the  poles. 

The  general  level  of  the  ocean  is  called  the  true  level ;  a  horizon 
tal  plane  at  any  point  is  called  the  apparent  level. 

Equilibrium  of  Liquids  in  Communicating  Vessels. 

?9.  When  a  liquid  is  contained  in  vessels  which  com 
municate  Avith  each  other,  it  will  be  in  equilibrium  if  its 

Explain  the  effect  of  the  centrifugal  force  on  the  form  of  a  level  surface.  What 
is  a,  true  level  f  An  apparent  level  ?  (79.)  What  are  the  conditions  of  equilibrium 
in  communicating  vessels? 


Si  POPULAR   PHYSICS. 

upper  surface  in  all  of  the  vessels  is  in  the  same  horizontal 
plane. 

This  principle  is  demonstrated  by  means  of  the  apparatus  repre 
sented  in  Fig.  51.  This  apparatus  consists  of  a  system  of  glass 
vessels  of  different  shapes  and  capacities,  all  of  which  communicate 
by  a  tube,  ac.  If  any  amount  of  water  or  other  liquid  be  poured 


Fig.  51. 

into  one  of  the  branches  and  allowed  to  come  to  rest,  it  will  be  seen 
that  its  upper  surface  in  all  of  the  vessels  is  in  the  same  horizontal 
plane.  The  reason  of  this  is,  obviously,  a  necessary  consequence  of 
the  principle  of  equal  pressures. 

Case  of  Vessels  containing    Liquids  of  different  Densities. 

8O.  When  liquids  of  different  densities  are  contained  in 
communicating  vessels,  they  will  be  in  equilibrium  when  the 
heights  of  the  columns  are  inversely  as  their  densities. 

This  principle  is  demonstrated  by  means  of  an  apparatus  shown  in 
Fig.  52.  The  apparatus  consists  of  two  glass  tubes,  A  and  B.  open 


How  is  this  demonstrated?    (  8O.)  What  are  the  conditions  of  equilibrium  'n  the 
case  of  liquids  of  different  densities  ?    How  is  this  demonstrated  ? 


EQUILIBRIUM   OF   LIQUIDS. 


85 


at  top,  and  communicating  at  bottom  by  a  smaller  tube,  If  a 
quantity  of  mercury  be  poured  into  one  of  the  tubes,  it  will  come  to 
a  level  in  both  tubes,  according  to  the  principle  explained  in  the 
preceding  article.  If  a  quantity  of  water  be  poured  into  the  tube  A, 
the  level  of  the  mercury  in  that  tube  will  be  depressed,  whilst  it  will 
be  elevated  in  the  tube  B.  The  difference  of  level,  t/c,  can  bo 
determined  by  the  graduated  scales  on  the  tubes.  It  will  be  found 
by  measurement,  that  the  column  of  water.  aZ»,  is  13.6  times  as  high 
as  the  column  of  mercury,  dc.  which  it  supports.  It  will  be  shown 
hereafter,  that  mercury  is  13.6  times  as  dense  as  water;  hence  the 
principle  is  proved.  Other  liquids  may  be  employed  with  similar 
results. 


Equilibrium  of  Heterogeneous   Liquids. 

81.     If  liquids  of  different  densities,  but  which  do  not 
mix,  be  poured  into  a  vessel,  they  will  arrange  themselves 

(81.)  What  are  the  conditions  of  equilibrium  of  heterogeneous  liquids? 


86 


POPULAR   PHYSICS. 


in  the  order  of  their  densities,  the  heaviest  being  at  the 
bottom,  and  the  upper  surface  of 
each  will  be  horizontal. 


This  is  shown  by  a  vial,  Fig.  53, 
containing  liquids  of  different  densities, 
as  mercury,  water,  and  oil.  If  the  vial 
be  shaken,  the  liquids  appear  to  mix, 
but  if  allowed  to  stand,  they  arrange 
themselves  in  horizontal  layers,  the 
densest  liquid  at  the  bottom. 

The  vial  in  the  figure  is  represented' 
as   containing   four   liquids.      It  was 
formerly  called  the  vial  of  four  de 
ments. 

It  is  in  accordance  with  this  princi 
ple  that  cream  rises  on  milk,  and  oil 
on  water.  The  principle  is  often 
employed  to  separate  liquids  of  dif 
ferent  density  by  the  process  of  decant 
ing. 


Fig  53. 


III. APPLICATIONS     OF     THE     PRINCIPLE     OF     EQUILIBRIUM. 

The   Water  LeveL 

82.  A  WATER  LEVEL  is  an  instrument  employed  for 
determining  the  difference  of  level  between  two  points.  It 
consists  of  a  horizontal  tube  of  tin,  2i  or  3  feet  in  length, 
into  the  extremities  of  which  two  glass  tubes  are  inserted 
perpendicular  to  it.  The  whole  rests  upon  a  three-legged 
support,  called  a  tripod,  as  shown  in  Fig.  54.  A  quantity 
of  water  tinged  with  carmine  or  other  coloring  matter  is 
introduced  into  one  of  the  glass  tubes,  which,  flowing 
through  the  horizontal  tube,  rises  to  the  same  level  in 
the  other.  A  visual  ray  directed  along  the  surfaces  of  the 


How  shown  f    (82.)  What  is  a  Water  Level?  Describe  it  and  its  use. 


APPLICATIONS. 


87 


water  in  the  two  glass  tubes  will  be  a  horizontal  line,  or  a 
line  of  apparent  level.  The  use  of  the  instrument  is  evident 
from  the  figure. 


• 


Tlje  Spirit  Level. 

83.  The  SPIRIT  LEVEL  consists  of  a  tube  of  glass  nearly 
filled  with  alcohol,  and  closed  at  its  two  extremities.  The 
tube  is  slightly  curved,  and  when  placed  horizontally,  the 


Fig.  55. 

bubble  of  air  which  it  contains  rises  to   the  middle  of  the 
upper   side   of  the  tube.     If  either  end  be  depressed,  the 

(83.)  Describe  a  Spirit  Level.     How  mounted?  , 


88 


POPULAE    PHYSICS. 


bubble  runs  towards  the  other  end. 
narily  mounted  in  a  wooden  case. 


When  used  it  is  ordi- 


This  form  of  level  is  much  used  by  masons,  carpenters,  and  other 
artisans.  To  ascertain  whether  a  surface  is  level,  the  instrument  is 
laid  upon  it,  and  the  position  of  the  bubble  noticed.  If  the  bubble  is 
in  the  middle  of  the  tube,  the  surface  is  level. 

This  form  of  level  is  also  attached  to  many  kinds  of  surveying  and 
astronomical  instruments. 


Fig.  56. 


Springs.  —  Fountains.  —  Rivers. 

84.  It  is  the  principle  of  equal  pressures  that  causes 
water  to  rise  in  springs  and  fountains.  The  water  which 
feeds  them  is  contained  in  natural  or  artificial  reservoirs 
higher  than  the  spring  or  fountain.  These  reservoirs  com 
municate  with  the  springs  or  fountains  by  natural  or  arti 
ficial  channels,  and  the  pressure  of  the  water  in  them 

What  are  its  uses  f    Applications.    (  84.)  What  is  a  Spring ?    Fountain  ? 


APPLICATIONS.  89 

causes  that  in  the  spring  or  fountain  to  boil  up,  or  sometimes 
to  shoot  up  in  a  jet. 

Fig.  56  represents  a  fountain  called  a  jet  cPeau.  The  reservoir  is 
on  the  hill  to  the  left,  and  the  water  reaches  the  bottom  of  the  basin 
by  a  pipe  represented  by  dotted  lines. 

The  water  of  the  jet  tends  to  rise  to  the  level  of  that  in  the  reser- 
voir.  and  would  do  so  were  it  not  for  the  resistance  of  the  air,  the 
friction  of  the  water  against  the  pipe,  and  the  resistance  offered  by 
the  falling  particles,  all  of  which  combine  to  render  the  jet  lower 
than  the  fountain-head. 

The  same  principle  determines  the  flow  of  streams  from  the  highef 
to  the  lower  grounds.  The  water  of  lakes,  seas,  and  oceans  is  con 
tinually  evaporating  to  form  vapors  and  clouds.  These  are  condensed 
in  the  form  of  rain,  and  the  particles  of  water,  urged  by  their  own 
weight,  seek  a  lower  level.  The  rivulets  gather  to  form  brooks,  and 
these  unite  to  form  rivers,  by  which  the  water  is  once  more  returned 
to  the  oceans  and  lakes.  All  of  the  water  does  not  flow  back  to  the 
ocean  along  the  surface,  but  a  portion  percolates  through  the  porous 
soils  and  accumulates  in  cavities  to  feed  our  springs  and  wells. 


Artesian  Wells. 

85.  AETESIAN  WELLS  are  deep  wells,  formed  by  boring 
through  rocks  and  strata  of  various  kinds  of  earth  to  reach 
a  supply  of  water.  These  wells  are  named  from  the  province 
of  Artois,  in  France,  where  they  were  first  used. 

Fig.  57  illustrates  the  principle  of  these  wells.  //  is  the  natural 
surface  of  the  earth.  AB  and  CD  are  curved  strata  of  clay  or 
rock  which  do  not  allow  of  the  percolation  of  water.  KK  is  an 
intermediate  stratum  of  sand  or  gravel,  which  permits  water  to 
penetrate  it.  When  a  hole.  7,  is  bored  down  to  strike  the  water 
bearing  stratum,  KK,  the  pressure  of  the  water  in  the  stratum  forces 
it  up  in  a  jet.  The  well  of  Grenelle,  in  Paris,  is  nearly  1800  feet 


Explain  the  jet  cPeau  f  What  causes  the  flow  of  streams  t  ffow  are  they  fed  f 
(85.)  What  are  Artesian  Wells?  Explain  their  action?  How  deep  is  that  at 
Paris  t 


POPULAK   PHYSICS. 


Fig.  57. 

deep,  and  is  fed  by  water  coming  from  the  hills  of  Champagne,  which 
are  much  higher  than  Paris.  The  supply  of  water  from  this  well  is 
immense. 

Many  Artesian  wells  have  been  sunk  in  our  own  country. 


IV.  —  PRESSURE      ON      SUBMERGED      BODIES. 

Principle  of  Archimedes. 

86.     IF  a  body  is  submerged  in  a  fluid,  it  will  be  pressed 
in  all  directions,  but  not  equally. 

To  illustrate,  suppose  a  cube 
immersed  in  water,  as  shown  in 
Fig.  58.  The  lateral  faces,  a  and 
6,  will  be  equally  pressed  and  in 
opposite  directions.  The  same  will 
be  true  for  the  other  lateral  faces. 
Hence,  the  horizontal  pressures  will 
exactly  neutralize  each  other.  The 
upper  and  lower  faces,  c  and  d:  will 
be  unequally  pressed,  and  in  oppo 
site  directions.  The  face,  c,  will  Fig.  58. 


(86.)    Are  submerged  bodies  pressed  equally  in  all  directions?     Illustrate  in 
detail. 


PRESSURE    ON   SUBMERGED   BODIES.  91 

be  pressed  upwards  by  a  force  equal  to  the  weight  of  a  column  of 
the  liquid  whose  cross-section  is  that  of  the  cube,  and  whose  height 
is  the  distance  of  c  from  the  surface  of  the  fluid.  The  face,  d:  will 
be  pressed  downwards  by  the  weight  of  a  column  of  the  liquid, 
having  the  same  cross-section  as  the  cube,  and  a  height  equal  to 
the  distance  of  d  from  the  surface  of  the  liquid ;  the  resultant  of 
these  two  pressures  is  an  upward  force,  equivalent  to  the  weight  of 
a  volume  of  the  liquid  equal  to  that  of  the  cube.  This  upward 
pressure  is  the  buoyant  effort  of  the  fluid. 

The  principle  just  explained  is  called  the  Principle  of 
Archimedes.  It  may  be  expressed  by  saying  that,  a  sub 
merged  body  loses  a  portion  of  its  weight  equal  to  that  of 
the  displaced  fluid. 


Hydrostatic  Balance. 

87.  A  HYDROSTATIC  BALANCE  is  a  balance  having  a 
hook  attached  to  the  lower  face  of  each  scale  pan,  and  so 
constructed  that  the  beam  may  be  raised  or  lowered  at 
pleasure. 

Fig.  59  represents  a  hydrostatic  balance.  The  cylinder,  c,  is  solid, 
and  fitted  to  slide  up  and  down  in  the  hollow  cylinder,  d.  The 
cylinder,  c,  may  be  confined  in  any  position  by  means  of  a  clarnp 


Cylinder  and  Bucket  Experiment. 

88.  The  principle  of  ARCHIMEDES  may  be  illustrated  by 
what  is  called  the  Cylinder  and  Bucket  Experiment^  as 
shown  in  Fig.  59.  A  hollow  cylinder  or  bucket,  #,  of 
brass,  is  attached  to  the  hook  of  one  of  the  scale  pans,  and 
from  it  is  suspended  a  solid  cylinder  of  brass,  just  large 
enough  to  fill  the  bucket,  and  the  two  are  balanced  by 
weights  placed  in  the  opposite  scale  pan.  A  glass  vessel 


Enunciate  the  principle  of  ARCHTMKDES.    (87.)  What  is  a  Hydrostatic  Balance? 
Explain  its  construction.    ( 88.)  Explain  the  Cylinder  and  Bucket  Experiment. 


92 


POPULAR     PHYSICS. 


having  been  placed  beneath  the  cylinder,  water  is  gradually 
poured  into  it,  until  the  cylinder  is  immersed.  The  oppo 
site  scale  pan  will  descend,  showing  that  the  cylinder  is 


Fig.  59. 

buoyed  up  by  some  force.  If  we  now  fill  the  bucket,  6, 
with  water,  the  equilibrium  will  be  restored,  and  the  beam 
will  come  to  a  level.  Because  the  water  poured  into  the 
bucket  is  equal  to  that  displaced  by  the  cylinder,  we  infer 
that  the  buoyant  effort  is  exactly  equal  to  the  weight  of 
the  displaced  fluid. 

The  principle  of  ARCHIMEDES  is  so  called,  because  it  was  first 
discovered  by  the  illustrious  philosopher  of  that  name.  He  was  led 
to  the  discovery  in  an  attempt  to  detect  a  fraud,  perpetrated  upon 


Why  is  the  principle  of  ARCHIMEDES  so  called  f 


PRESSURE   ON    SUBMERGED   BODIES.  93 

HIERO  of  Syracuse,  by  a  goldsmith,  who  had  been  employed  to  make 
a  golden  crown.  The  artisan  mixed  a  portion  of  silver  with  the 
gold  that  was  given  him  for  making  the  crown,  but  by  means  of  the 
principle  above  explained,  ARCHIMEDES  wras  able  to  determine  the 
exact  amount  of  each  material  employed. 

Floating  Bodies.— Principles  of  Flotation. 

89.  When  a  body  is  plunged  into  a  liquid,  it  is  urged 
downward  by  its  proper  weight,  and  upward  by  the  buoyant 
effort  of  the  liquid,  and,  according  to  the  relative  intensities 
of  these  two  forces,  three  cases  may  arise  : 

1.  If  the   density  of  the  immersed  body  is  the  same  as 
that  of  the  liquid,  its  weight  will  be  equal  to  the  buoyant 
effort  of  the  liquid,  and  it  will  remain  in  equilibrium  wher 
ever  it  may  be   placed.     This  is  practically  the  case  with 
fishes.     They  maintain  themselves  in  any  position  in  which 
they  may  happen  to  be,  without  effort. 

2.  If  the  density  of  the  body  is  greater  than  that  of  the 
liquid,  its  weight  will  be  greater  than  the  buoyant  effort, 
and  the  body  will  sink  to  the  bottom.     This  is  what  hap 
pens  when  a  stone  or  piece  of  iron  is  thrown  into  water. 

3.  If  the  density  of  the  body  is  less  than  that  of  the  liquid, 
its  weight  will  be  less  than  the  buoyant  effort,  and  the  body 
will  rise  to  the  surface.     The  body  will  continue  to  rise 
until  the  weight  of  the  displaced  liquid  equals  that  of  the 
body,  when  it  will  come  to  rest.     It  is  then  said  to  float. 
Thus,  a  piece  of  wood  floats  upon  water,  and  in  like  manner 
a  piece  of  iron  floats  upon  mercury. 

When  a  floating  body  comes  to  rest  on  a  liquid,  the 
plane  of  the  upper  surface  of  the  liquid  is  called  the  Plane 
of  Flotation. 

Explain  -the  method  of  Us  discovery.    (  89.)  When  a  body  is  plunged  into  a  liquid, 
what  three  cases  may  arise  ?     Explain  the  first  case.    The  second  case.    The  third 

case.     V7h.it  i ;  !!::>  ?!.;:•.•  <>: Flotation ? 


94 


POPULAR     PHYSICS. 


It  sometimes  happens  that  a  body  which  is  more  dense  than  a 
liquid  floats  upon  it.  Thus,  a  porcelain  saucer  floats  upon  water. 
This  arises  from  its  form  being  such,  that  it  displaces  its  own  weight 
of  water,  when  only  partially  immersed.  For  the  same  reason  iron 
ships  float  freely  on  the  ocean. 

Illustration  of  the  Principles  of  Flotation. 

9O.  The  principles  of  flotation  may  be  illustrated  by  an  instru 
ment  shown  in  Fig.  60,  which  under  various  formg  is  sold  in  the 
shops  as  a  child's  toy. 

In  the  form  shown,  it  consists  of 
a  high  and  narrow  glass  vessel,  sur 
mounted  by  a  brass  cylinder,  A.  in 
which  is  an  air-tight  piston  that 
may  be  raised  or  depressed  by  the 
hand.  The  vessel  is  partially  filled 
with  water,  and  contains  a  light 
body,  as  a  fish,  hollow  and  of  porce 
lain  or  glass.  The  fish  is  attached  to 
a  sphere  of  glass,  m:  filled  with  air, 
and  with  a  small  hole,  o,  at  its 
lower  side,  through  which  water 
can  flow  in  or  out,  as  the  pressure 
is  increased  or  diminished. 

Under  ordinary  circumstances  the 
sphere,  m,  with  its  attached  fish, 
floats  at  the  surface  of  the  water. 
If  the  piston  is  depressed,  the  air 
beneath  it  is  compressed,  and  acting 
upon  the  water  forces  a  portion  of 
it  into  the  globe.  The  apparatus 
then  becomes  more  dense  than  the 
water,  and  sinks.  By  relieving  the 
pressure,  the  air  in  the  globe  expands  and  drives  the  water  out, 
when  it  again  floats  on  the  surface.  The  experiment  may  be  re 
peated  at  pleasure. 


Fig.  60 


Explain  the  case  of  a  dense  body  floating  on  a  liquid.    (9O.)  What' instrument 
illustrates  the  laws  of  flotation  f    Explain  its  use  and  action. 


PRESSURE    ON   SUBMERGED   BODIES.  95 


Swimming  Bladder  of  Fishes.' 

91.  In  many  fishes  there  is  a  bladder  filled  with  air, 
situated  directly  under  the  backbone.  This  is  called  the 
Swimming  Bladder. 

When  the  fish  wishes  to  descend,  it  compresses  this  bladder  by  a 
muscular  effort,  and  then,  as  the  quantity  of  water  displaced  is  less 
than  before,  the  weight  of  the  fish  prevails  over  the  buoyant  effort, 
and  the  fish  sinks.  On  relaxing  the  effort,  the  bladder  expands,  the 
buoyant  effort  of  the  water  prevails  over  the  weight  of  the  fish,  and 
it  rises. 


Fig.  61. 


Swimming. 

92.  The  human  body  is  lighter  than  water,  especially  than  the 
salt  water  of  the  ocean,  and  tends  naturally  to  float  when  immersed. 
The  only  reason  why  men  do  not  swim  naturally,  is  the  difficulty 
of  keeping  the  head  out  of  water,  so  as  to  be  able  to  breathe.  The 
head  is  the  heaviest  part  of  the  body,  and  tends  continually  to  sink 
into  the  water. 

Many  quadrupeds  swim  naturally,  because  the  head  is  small  in 
proportion  to  the  body,  and  is  so  placed  upon  the  trunk,  that  it  is 
easy  to  keep  it  above  the  surface. 

The  safest  position  for  a  person  in  the  water,  who  does  not  know 

(  91 .)  What  is  the  Swimming  Bladder  of  a  fish  ?  Explain  its  acUon.  ( 92 .)  Ex 
plain  the  phenomenon  of  swimming.  Why  do  some  quadrupeds  swim  naturally  ? 
"What  is  the  safest  position  in  the  water  t 


96  POPULAR     PHYSICS. 

how  to  swim,  is  upon  the  back.  The  tendency  to  raise  the  arms  out 
of  the  water  should  be  resisted,  as  this  diminishes  the  buoyant  effort 
of  the  fluid  without  diminishing  the  weight. 

In  learning  to  swim,  it  is  often  the  custom  to  place  bladders  filled 
with  air,  or  blocks  of  cork,  under  the  arms,  as  shown  in  Fig.  61. 
These  act  to  increase  the  buoyant  effort  of  the  fluid,  without  sen 
sibly  increasing  the  weight.  It  is  on  this  principle  that  life- 
preservers  are  constructed. 

Many  kinds  of  birds,  as  ducks,  geese,  swans,  and  the  like,  swim 
naturally  and  without  effort.  They  owe  this  faculty  to  a  thick 
layer  of  down  and  feathers  which  are  very  light  and  impermeable 
by  water.  They,  therefore,  displace  a  large  volume  of  water  in 
proportion  to  their  weight,  giving  rise  to  a  strong  buoyant  effort. 


V.  — SPECIFIC   GRAVITY   OF   BODIES. 

Definition  of  Specific  Gravity. 

93.  The  SPECIFIC  GRAVITY  of  a  body  is  its  relative 
weight ;  that  is,  it  is  the  number  of  times  the  body  is 
heavier  than  an  equivalent  volume  of  some  other  body 
taken  as  a  standard. 

It  is  a  matter  of  daily  observation,  that  some  substances  are 
heavier  than  others  under  the  same  volume.  Thus,  gold  is  heavier 
than  silver,  lead  than  iron,  stones  than  wood,  and  so  on.  In  order 
to  compare  the  relative  weights  of  different  bodies,  all  are  referred 
to  a  common  standard. 

Distilled  water  is  generally  adopted  as  a  standard,  and  because 
water  varies  in  density  at  different  temperatures,  it  is  usual  to  take 
it  at  the  temperature  of  39°.2  Fahrenheit,  water  being  most  dense 
at  that  temperature. 

In  order  to  find  the  specific  gravity  of  any  body,  all  that 
we  have  to  do  is,  to  find  how  many  times  heavier  any 

What  is  the  principle  of  the  life-preserver  f  Why  do  some  birds  swim  natur 
ally  f  (  93.)  What  is  Specific  Gravity?  Illustrate.  What  is  taken  as  a  standard  f 
At  what  temperature?  Why?  What  is  the  process  of  finding  the  specific  gravity 
of  a  body  ? 


SPECIFIC   GRAVITY. 


97 


given  volume  of  the  body  is,  than  an  equivalent  volume 
of  distilled  water  at  39°.2  F.  This  is  the  method  of  fixing 
the  specific  gravity  of  solids  and  liquids  ;  we  shall  see  here 
after  how  it  is  possible  to  fix  the  specific  gravity  of  gases 
and  vapors. 


Fig.  62. 


Specific  Gravity  of  Solids. 

94.  The  following  are  some  of  the  methods  of  determin 
ing  the  specific  gravities  of  solids : 

1.  By  the  Hydrostatic  Balance. — Place  the  body  in  one 
of  the  scale  pans  and  balance  it  by  known  weights  in  the 
other  pan.  These  will  give  the  weight  of  the  body  in  air. 

(94.)  Explain,  in  detail,  the  method  of  finding  the  specific  gravity  of  a  solid  by 
means  of  the  hydrostatic  balance. 

5 


98  POPULAR    PHYSICS. 

Next  suspend  the  body  in  a  vessel  of  distilled  water  oy 
means  of  a  thread  or  wire  attached  to  one  of  the  scale  pans, 
as  shown  in  Fig.  62,  and  balance  it  by  weights  placed  in  the 
other  pan.  On  account  of  the  buoyant  effort  of  the  water, 
the  weight  of  the  body  in  water  will  be  less  than  that  in  air. 
Subtract  the  weight  of  the  body  in  water  from  that  in  air, 
and  the  difference  will  be  the  weight  of  the  displaced  water, 
that  is,  the  weight  of  a  volume  of  water  equal  to  that  of  the 
body.  Having  found  the  weight  of  the  body  in  air,  and 
the  weight  of  an  equivalent  volume  of  water,  divide  the 
former  by  the  latter,  and  the  result  will  be  the  specific 
gravity  required. 

2.  By  N'icholsoii's  Hydrometer. — NICHOLSON'S  HYDROME 
TER  consists  of  a  hollow  cylinder  of  glass,  as  shown  in  Fig.  G3, 
weighted  at  the  bottom  by  a  heavy  body,  c?,  to  make  it  float 
erect,  and  terminating  above  by  a  thin  stem,  c,  which  sup 
ports  a  scale  pan,  a.  The  instrument  is  so  constructed  that 
when  a  given  weight,  say  500  grains,  is  placed  in  the  pan, 
it  will  sink  in  distilled  water  to  a  notch,  c,  on  the  stem. 

The  method  of  determining  the  specific  gravity  by  means 
of  this  instrument  is  shown  in  Figs.  64  and  65.  Suppose 
it  w^ere  required  to  determine  the  specific  gravity  of  a  small 
bar  of  iron  weighing  less  than  500  grains. 

The  bar  is  placed  in  the  pan  and  weights  added  till  it 
sinks  to  the  notch  in  the  stem  as  shown  in  Fig.  64.  These 
weights,  subtracted  from  500  grains,  give  the  weight  of  the 
bar  in  air.  Next  place  the  bar  in  the  cup,  d,  as  shown  in 
Fig.  65,  and  add  weights  enough  to  make  the  instrument 
sink  again  to  the  notch  in  the  stem.  The  last  weights 
will  denote  the  buoyant  effort  of  the  fluid,  or  the  weight  of 
the  water  displaced  by  the  bar.  Div  de  the  weight  of  the 
bar  in  air  by  the  weight  of  the  displaced  water,  and  the 
result  will  be  the  specific  gravity  sought. 

What  is  Nicholson's  Hydrometer  ?  How  used  for  determining  the  specific  gravity 
of  a  solid  ? 


SPECIFIC    GRAVITY. 


99 


3.  By  a  flask. — This  method  is  used  when  a  body  exists 
in  a  state  of  powder,  or  in  fine  particles  like  sand.  A  small 
flask,  whose  exact  weight  is  known,  is  first  filled  with  the 
powder  and  the  whole  carefully  weighed.  The  entire  weight, 
diminished  by  that  of  the  flask,  is  the  weight  of  the  body. 


Fig.  63. 


Fig.  64. 


Fig.  65. 


The  flask  is  then  filled  with  water  and  weighed.  This 
weight,  diminished  by  that  of  the  flask,  is  the  weight  of  an 
equivalent  volume  of  water.  Divide  the  weight  of  the  body 
by  that  of  its  equivalent  volume  of  water,  and  the  result  will 
be  the  specific  gravity  required. 

Specific   Gravity   of  Liquids. 

95.     The  following  are  some  of  the  principal  methods  of 
determining  the  specific  gravities  of  liquids : 

1.  By  the  Hydrostatic  Balance. — Select  a  heavy  body 
which  is  not  soluble  either  in  water  or  in  the  liquid  whose 

Explain  the  method  by  means  of  a  flask.    (95.)  How  is  the  specific  gravity  of  a 
liquid  found  by  means  of  the  balance  ? 


100 


POPULAR     PHYSICS. 


specific  gravity  is  to  be  determined,  as,  for  example,  a  ball 
of  platinum.  Weigh  this  body  first  in  air,  then  in  water, 
and  finally  in  the  liquid  in  question.  Subtract  the  second 
and  third  weights  from  the  first  separately ;  the  results 
obtained  will  be  respectively  the  weights  of  a  volume  of 
water,  and  of  the  liquid,  equal  to  that  of  the  platinum  ball. 
Divide  the  latter  by  the  former,  and  the  quotient  will  be  the 
specific  gravity  required. 


Fig.  66. 

2.  By  Fahrenheit'' s  Hydrometer. — FAHRENHEIT'S  HYDRO 
METER  consists  of  a  glass  cylinder  ballasted  at  the  bottom  by 
a  small  globe  filled  with  mercury,  and  provided  at  top  with 
a  stem  and  scale  pan  as  shown  in  Fig.  66.  Its  weight  is 
carefully  determined. 

To  use  the  hydrometer,  it  is  first  plunged  into  distilled 
water,  and  weights  placed  in  the  scale  pan  till  it  sinks  to  the 


Describe  Fahrenheit's  Hydrometer.    How  is  it  used  to  find  the  specific  gravity 
of  a  liquid? 


SPECIFIC    GltAVITY.  101 

notch  filed  on  the  stem.  These  weights,  increased  by  that 
of  the  instrument,  will  give  the  weight  of  the  displaced  water. 
The  instrument  is  next  plunged  into  the  liquid  in  question, 
and  weights  are  placed  in  the  pan  till  the  instrument  again 
sinks  to  the  notch.  These  weights,  added  to  that  of  the 
instrument,  give  the  weight  of  the  displaced  liquid.  Now 
the  volumes  displaced  are  the  same  in  both  cases,  each  being 
that  of  the  submerged  instrument ;  hence,  if  we  divide  the 
weight  of  the  displaced  liquid  by  that  of  the  displaced  water, 
the  quotient  will  be  the  specific  gravity  required. 

3.  By  the  flask. — A  flask  is  constructed  so  as  to  hold  a 
given  weight  of  distilled  water,  say  1000  grains.  This  flask 
is  first  weighed  when  empty,  and  then  when  filled  with  the 
liquid  in  question.  The  difference  of  these  results  is  the 
weight  of  the  liquid,  and  this,  divided  by  1000  grains,  will 
be  the  specific  gravity  required. 

The  specific  gravities  of  some  of  the  most  important  substances  are 
given  in  the  following  table : 

TABLE, 

SHOWING    THE    SPECIFIC    GRAVITIES    OF    SOLIDS    AND    LIQUIDS. 


Platinum  (rolled) 22.07 

Gold  (stamps) 19.36 

Lead  (cast) 11.35 

Silver  (cast) 10.47 

Iron  (bar) 7.79 

Zinc  (cast) 6.86 

Diamond 3.53 

White  Marble 2.84 

Glass  (flint) 3.33 

Ivory 1.92 


Mercury 13.60 

Sulphuric  Acid 1.84 

Milk 1.03 

Sea  Water 1.03 

Distilled  Water 1.00 

Bordeaux  Wine 0.99 

Olive  Oil •.91 

Spirits  of  Turpentine  ...  0.87 

Absolute  Alcohol 0. 79 

Ordinary  Ether 0. 72 


It  will  be  seen  that  platinum  is  the  heaviest  solid,  and  that  mer 
cury  is  the  heaviest  liquid. 

How  IB  the  specific  gravity  of  a  liquid  determined  by  means  of  a  flask  ?     Which 
is  (he  heaviest  solid  ?    Liquid  f 


102 


POPULAR     PHYSICS. 


A  knowledge  of  the  specific  gravities  of  bodies  is  of  frequent 
application.  In  mineralogy  it  aids  in  determining  mineral  species. 
The  jeweller  determines  by  its  aid  the  precious  stones.  It  enables 
us  to  find  the  weight  of  a  body  when  we  know  its  volume.  Thus,  a 
cubic  foot  of  iron  weighs  11.35  times  as  much  as  a  cubic  foot  of 
water  ;  but  a  cubic  foot  of  water  weighs  1000  ounces,  hence  a  cubic 
foot  of  iron  weighs  11,350  ounces,  or  about  709  Ibs. 

Beaume's  Areometer. 

96.  BEAUME'S  AREOMETER  consists  of  a  bulb  of  glass, 
ballasted  at  bottom  by  a  second  bulb  containing  mercury, 
and  terminating  at  top  in  a  cylinder  of  uniform  diameter,  as 
shown  in  Fig.  67. 

When  plunged  into  liquids,  it 
sinks  till  the  weight  of  the  dis 
placed  fluid  equals  that  of  the 
areometer.  In  light  fluids  it  there 
fore  sinks  deeper  than  in  heavy 
ones. 

The  plan  of  graduating  BEAUME'S 
areometer  is  as  follows.  It  is  bal 
lasted  so  that  in  distilled  water 
it  will  sink  to  the  point  a,  on  the 
stem,  which  is  marked  0.  A  mix 
ture  of  salt  and  pure  water  is  then 
formed,  in  the  proportion  of  15  of 
the  former  to  85  of  the  latter,  into 
which  the  instrument  is  plunged. 
The  upper  surface  then  cuts  the 

stem  at  some  point,  c,  which  is  marked  15.  The  interme 
diate  space  between  a  and  c,  is  divided  into  15  equal  parts, 
and  the  division  is  continued  downwards  on  the  stem.  The 
divisions  and  numbers  are  on  a  slip  of  paper  in  the  interior 
of  the  stem. 


Fig.  67. 


What  are  some  of  the  applications  of  the  specific  gravity  of  bodies  ?   (  96 .)  Des 
cribe  BEAUME'S  Areometer  ?    How  is  it  graduated? 


SPECIFIC   GRAVITY. 


103 


The  use  of  the  instrument  thus  graduated  is  to  ascertain 
the  amount  of  salt  in  any  solution  of  salt  in  water.  It  is 
plunged  into  the  solution  in  question,  and  the  number  to 
winch  it  sinks,  denotes  the  degree  of  saturation  of  the  solu 
tion. 

Instruments  constructed  on  this  principle  have  been  devised  for 
determining  the  strength  of  other  solutions,  whether  of  acids  or 
palts.  Also  for  determining  the  strength  of  saccharine  solutions  and 
the  like. 

The  Alcoholometer. 

97.  The  ALCOHOLOMETER  is  similar  in  its  construction  to 
the  areometer  just  described.  It  is  graduated  so  as  to  show 
the  percentage  of  alcohol  in  any  mixture  of  alcohol  and 
water. 

The  instrument  is  first  ballasted  so 
that  when  plunged  in  pure  water  it  will 
float  with  nearly  all  of  its  stem  above 
the  water.  The  line  of  flotation  is 
marked  0.  Mixtures  are  then  formed, 
containing  1,  2,  3,  <fcc.,  per  cent,  of  pure 
alcohol  and  water,  and  the  instrument 
is  plunged  injto  them  in  succession.  The 
lines  of  flotation  are  marked  1,  2,  3,  <fcc., 
as  in  the  instrument  previously.  In  this 
case  the  numbers  run  upwards.  It  is 
necessary  to  graduate  it  throughout  by 
trial,  as  the  divisions  are  not  uniform. 

To  use  the  instrument,  it  is  plunged 
into  the  mixture  of  alcohol  and  water  to  be  tested,  and  the 
per-centage  is  read  off  on  the  paper  scale  within  the  tube,  or 
else  the  scale  is  scratched  upon  the  stem  with  a  diamond. 


Fig.  68. 


What  is  its  use  ?    How  used  ?    What  other  instruments  are  constructed  on  the  same 
principle ?    (97.)  Describe  the  Alcoholometer.     How  is  it  graduated  ?    How  used  ? 


134  POPULAR   TIIYSICS. 


The   Lactometer. 

* 

98.     The  LACTOMETER  is  entirely  analogous  in  principle 
to  BEAUME'S  areometer,  and  is  used  to  determine  the  purity 
of  milk.    The  instrument,  and 
the  method  of  using  it,  are 
shown  in  Fig.  69.  ^  ^9       A 

It  is  graduated  by  trial, 
using  mixtures  of  milk  and 
water.  In  the  first  trial  pure 
water  is  used,  then  mixtures 
containing  10,  20,  30,  40,  &c., 
per  cent,  of  milk.  The  scale  Fig>  69> 

is  therefore  divided  into  10  parts,  between  pure  water  and 
pure  milk. 

(  98.)  What  is  a  Lactometer  ?    How  graduated  and  used? 


CHAPTER    III. 

MECHANICS       OF      GASES       AND      VAPORS. 
I.  —  THE     ATMOSPHERE. 

General  Properties  of  Gases  and  Vapors. 

99.  GASES  and  VAPORS  have  been  denned  to  be  highly 
compressible  fluids. 

The  distinction  between  a  gas  and  a  vapor,  is  not  very  clear. 
When  a  body  in  a  gaseous  form  can,  by  moderate  pressure,  be  re 
duced  to  a  liquid  form,  it  is  usually  called  a  vapor.  For  most  of 
the  purposes  of  Physics  the  distinction  is  unimportant. 

Besides  the  property  of  compressibility,  or  rather  as  a 
consequence  of  it,  gases  and  vapors  continually  tend  to 
expand  so  as  to  occupy  a  greater  space.  The  force  which 
they  exert  in  this  way,  is  called  their  Tension,  or  their 
Elastic  Force. 

Thirty-four  gases  are  known,  thirty  of  which  are  compound,  and 
four  are  simple.  The  four  simple  gases  are.  oxygen,  hydrogen,  ni 
trogen,  and  chlorine.  Most  of  the  gases  are  colorless,  but  some  are 
not  so. 

Of  the  thirty-four  gases,  all  but  five  have  been  liquefied  by  pres 
sure,  and  the  application  of  cold.  The  five  that  have  thus  far  re 
sisted  are,  oxygen,  hydrogen,  nitrogen,  deutoxyde  of  nitrogen,  and 
carbonic  oxyde. 


(99.)  What  are  Gases  and  Vapors?  What  is  the  difference  between  them? 
What  is  meant  by  Tension?  How  many  known  gases  are  there?  Which  are 
simple  f  Which  have  not  been  liquefied  f 

5% 


106  POPULAR   PHYSICS. 

Description  of  the  Atmosphere. 

100.  The  air  we  breathe  is  a  mixture  of  oxygen  and 
nitrogen,  with  a  slight  quantity  of  carbonic  acid,  watery 
vapor,  and  some  accidental  impurities.      The  oxygen  and 
nitrogen  are  mixed  in  the  proportion  of  21  to  79. 

The  oxygen  of  the  air  supports  life  and  combustion ;  without  it, 
neither  could  long  exist.  The  nitrogen  serves  to  dilute  it.  Were 
the  air  composed  entirely  of  oxygen,  bodies  would  burn  with  too 
much  rapidity,  even  many  of  the  metals  would  be  consumed.  Ani 
mal  life,  too,  would  soon  be  exhausted  by  overaction  in  such  an 
atmosphere. 

The  atmosphere  is  transparent,  without  odor,  and  color 
less,  except  when  seen  in  masses.  In  masses,  it  assumes  a 
blue  tint,  and  it  is  this  which  causes  the  sky  to  take  a  blue 
color. 

Without  an  atmosphere,  the  celestial  vault  would  appear  perfectly 
black  ]  in  ascending  high  mountains,  the  sky  gradually  loses  its 
blueness,  and  approaches  a  hue  of  black;  this  is  because  the  mass 
of  air  above  the  observer  rapidly  diminishes  as  we  ascend. 

The  air,  by  virtue  of  its  elasticity,  serves  as  a  medium 
for  the  transmission  of  sound ;  it  also  serves  as  a  means  of 
transporting  the  vapors  of  oceans  and  lakes  to  fall  upon  the 
land  in  the  form  of  rain,  snow,  and  the  like. 

Expansive  Force  of  Air. 

101.  Air,  like  simple  gases,  always  tends  to  assume  a 
greater  volume. 

To  show  this  property,  take  a  bladder  fitted  with  a  stop-cock,  as 
shown  in  Fig.  70.  Having  moistened  the  bladder  to  make  it  more 
flexible,  open  the  cock,  squeeze  out  most  of  the  air,  and  then  close  it. 

(  1 00.)  Describe  the  composition  of  the  atmosphere.  What  is  the  use  of  the 
oxygen  f  Of  the  nitrogen  ?  What  is  the  color  of  air  ?  What  effect  has  the  air  on 
celestial  appearances?  Mention  some  of  the  uses  of  the  atmosphere.  (101.)  How 
is  the  expansive  force  of  air  shown  P 


THE   ATMOSPHERE. 


107 


Place  the  nearly  empty  bladder  under  the  receiver  of  an  air-pump, 
and  exhaust  the  air.  As  the  air  becomes  rarer  in  the  receiver,  the 
bladder  will  be  seen  to  expand,  showing  that  the  air  within  it  is 
expansible.  In  the  same  way,  it  may  be  shown  that  any  gas  is 
expansible. 


Fig.  70. 


Fig.  71. 


Weight  of  Air. 
1O2.     Air,  like  other  bodies,  has  weight. 

To  show  this,  take  a  hollow  globe  of  glass,  fitted  with  a  stop-cock, 
as  shown  in  Fig.  71.  Having  attached  it  to  one  scale  pan  of  a  deli 
cate  balance,  counterpoise  it  by  weights  placed  in  the  other.  Then 
by  means  of  the  air-pump  exhaust  the  air  from  the  globe  j  the  oppo 
site  scale  pan  will  descend,  and  some  weights  wdll  have  to  be  added 


( 1 02.)  How  is  it  shown  that  air  has  iveight. 


108  POPULAR   PHYSICS. 

to  the  first  scale  pan  to  restore  the  equilibrium      The  weights  added 
will  indicate  the  weight  of  the  exhausted  air. 

Composition  of  the  Atmosphere. 

103.  It  has  been  stated  that  our  atmosphere  is  com 
posed  principally  of  oxygen  and  nitrogen,  with  small  quanti 
ties  of  carbonic  acid  and  watery  vapor. 

The  amount  of  watery  vapor  depends  upon  the  place,  the 
season,  the  temperature,  and  the  direction  of  the  wind ; 
under  ail  circumstances  it  forms  but  a  small  per-centage  of 
the  entire  atmosphere. 

The  carbonic  acid  in  the  atmosphere  arises  in  a  great 
measure  from  respiration  and  combustion.  A  continual 
supply  of  this  gas  is  afforded  by  volcanoes.  On  the  other 
hand,  it  is  being  continually  taken  up  in  the  process  of  vege 
tation.  Plants  continually  absorb  it,  appropriating  the  car 
bon,  and  giving  off  the  oxygen  which  it  contains.  Another 
cause  of  diminution  in  the  amount  of  carbonic  acid  in  the 
air,  is  absorption  by  the  water  of  our  streams.  Water  ab 
sorbs  large  quantities  of  it,  which  thus  becomes  the  means 
of  dissolving  earthy  matters,  and  eventually  of  causing  cal 
careous  deposits. 

It  is  the  result  of  observation,  that  the  supply  and  loss 
are  very  nearly  balanced,  so  that  the  per-centage  of  carbonic 
acid  in  the  atmosphere  remains  nearly  constant.  It  amounts 
to  about  a  thousandth  part  of  the  entire  atmosphere. 

Atmospheric  Pressure. 

104.  The  atmosphere,  by  virtue  of  its  weight,  exerts  a 
force  of  pressure  upon  the  surface  of  the  earth  as  well  as 
upon  every  object  with  which  it  is  in  contact.     This  force 
is  called  the  Atmospheric  Pressure. 


(1O3.)  Upon  what  circumstances  does  the  watery  vapor  in  the  air  depend? 
Whence  is  carbonic  acid  supplied  ?  What  becomes  of  the  excess  of  carbonic  acid  ? 
How  do  the  supply  and  loss  compare?  "What  is  the  amount  in  the,  atmosphere? 

1O4.)  What  is  the  Atmospheric  Pressure  ? 


THE    ATMOSPHERE. 


109 


This  pressure  decreases  as  we  ascend  into  the  atmos 
phere. 

If  we  suppose  the  atmosphere  to  be  divided  into  layers  parallel  to 
the  surface  of  the  earth,  it  is  evident  that  each  layer  is  pressed  down 
by  the  weight  of  all  above  it.  Hence,  the  higher  layers  are  less 
compressed  than  those  below  them.  Being  less  compressed,  they 
expand,  or  become  rarefied.  The  existence  of  atmospheric  pressure 
may  be  shown  by  a  variety  of  experiments,  some  of  which  will  be 
explained  below. 


Fig.  72. 
Bursting  a  Membrane. 

1O5.     A  glass  cylinder,  open  at  both  ends,  has  its  upper  end 
covered  by  a  stretched  membrane,  such   as  is  used  by  gold-beaters, 

How  does  it  vary  as  we  ascend?     Row  shoton  that  the  air  becomes  rarer  in 
ascending  f    (1O5-)  Explain  the  experiment  of  bursting  a  membrane. 


110 


POPULAR     PHYSICS. 


and  its  lower  end  is  ground  so  as  to  fit  the  plate  of  an  air-pump,  as 
shown  in  Fig.  72. 

In  its  natural  condition,  the  membrane  is  pressed  down  by  the 
weight  of  the  atmosphere  above  it,  and  this  pressure  is  resisted  by 
the  tension  of  the  air  within  the  cylinder.  If  now  the  air  be  ex 
hausted  from  the  cylinder,  the  membrane  will  no  longer  be  pressed 
from  within,  and  will  finally  burst  with  a  loud  report. 

The  bursting  of  the  membrane  shows  the  pressure  of  the  air.  The 
report  arises  from  the  sudden  rush  of  air  to  fill  up  the  exhausted 
cylinder. 

The  Magdeburg  Hemispheres. 

1O6*  This  apparatus,  named  from  the  city  where  it  was  in 
vented,  consists  of  two  hollow  hemispheres  of 
brass,  which  are  ground  so  as  to  fit  each  other 
with  an  air-tight  joint.  The  hemispheres  are 
shown  in  Fig.  73.  One  of  them  is  so  prepared 
that  it  can  be  attached  to  an  air-pump,  and  is 
provided  with  a  stop-cock,  by  means  of  which 
a  communication  with  the  external  air  can  be 
opened  or  closed  at  pleasure. 

The  two  hemispheres  being  placed  one  upon 
the  other,  the  pressure  of  the  external  air  is 
exactly  counterbalanced  by  the  tension  of  that 
within,  and  no   obstacle  prevents   them  from 
being  drawn  apart.     If,  however,  the  air  be 
exhausted  from  within,  the  external  pressure 
is   no   longer   counteracted   by  an   expansive 
force  from  within,  and  it  requires  a  consider 
able  effort  to  effect  their  separation,  as  shown  in  Fig.  74.     We  shall 
see  hereafter  that  the  hemispheres  are  pressed  together  by  a  force 
equal  to  15  Ibs.,  multiplied  by  the  number  of  square  inches  in  their 
common  cross  section. 


FLr  73. 


What  causes  the  bursting?     The  report f    (106)  What  are  the  Magdebourg 
Hemispheres  f    Describe  the  experiment,  and  explain  it. 


THE    ATMOSriiEBE. 


Ill 


I   I 


The  experiment  was  devised  by  OTTO  VONGUERICKE,  of  MaL'de- 
bourg.  He  constructed  two  hemispheres  more  than  two  feet  in 
diameter,  and  after  having  exhausted  the  air,  it  is  reported  that 
it  required  several  horses  to  draw  them  asunder. 


Torricellian  Tube.— Measure  of  the  Atmospheric  Pressure. 

1O7.  The  preceding  experiments  show  that  the  atmos 
phere  exerts  a  force  of  pressure ;  the  intensity  of  that  force 
may  be  measured  by  other  means. 

TORKICELLI,  a  pupil  of  GALILEO,  showed  in  1643,  that  this 
pressure  amounts  to  about  15  Ibs.  on  each  square  inch  of 
surface,  at  the  level  of  the  sea. 


What  experiment  was  made  by  OTTO  DE  GTTJEKICKE?    (  1O7-)  What  is  the  pres 
sure  of  the  atmosphere  on  a  square  inch  ? 


112 


POPULAll    PHYSICS. 


In  order  to  repeat  TORRICELLI'S  experiment,  take  a  glass  tube 
about  three  feet  in  length,  closed  at  one  end  and  open  at  the  other. 
Turning  the  closed  end  downwards, 
let  it  be  filled  with  mercury.  Then 
holding  the  finger  over  the  open 
end,  let  it  be  inverted  in  a  vessel 
of  mercury,  as  shown  in  Fig.  75. 
On  removing  the  finger,  the  mer 
cury  sinks  in  the  tube  until  the 
column.  AB,  is  about  30  inches 
high,  when  it  comes  to  a  state  of 
equilibrium. 

In  this  condition,  the  mercury 
is  sustained  by  the  pressure  of 
the  air  upon  the  surface  of  the 
free  mercury  in  the  vessel,  trans 
mitted  according  to  the  law  ex 
plained  in  Article  70.  At  the 

level  of  the  sea,  the  height  of  the 

column  AB,  is  on  an   average  not 

far  from  30  inches,  or  2£  feet. 
If  we  suppose  the  cross- section 

of  the  tube  to  be  one  square  inch, 

the  atmospheric  pressure  upon  that 

surface  must  be  sufficient  to  bal 
ance  the  weight  of  30  cubic  inches 

of  mercury.     Now  the  weight  of 

30   cubic  inches  of  mercury  is  a 

little  less  than  15  Ibs. ;  hence,  we 

say  the  measure  of  the  atmospheric 

pressure  is   15  Ibs.  on  each  square 

inch. 


Fig.  75. 


A  pressure  of  15  Ibs.  on  each  square  inch,  is  often  called 
an  atmosphere,  and  this  becomes  a  unit  for  expressing  the 
pressures  of  gases  and  vapors.  Thus,  when  we  say,  in  any 
given  case,  that  the  pressure  of  steam  in  a  boiler  is  four 


Describe  TORRICELLI'S  experiment.     How  shown  that  the  pressure  is  15  Ibs.  on 
an  inch?    "What  unit  of  pressure  is  adopted  for  all  gases  and  vapors?    Example. 


THE    ATMOSPHERE.  113 

atmospheres,  we  mean  that  it  exerts  a  pressure  of  60  Ibs.  on 
each  square  inch  of  surface. 

Pascal's   Experiments. 

108.  As  soon  as  TORRICELLI'S  experiment  was  known 
in  France,  BLAISE  PASCAL  undertook  to  ascertain  by  experi 
ment  whether  the  mercury  was  actually  retained  in  the 
tube  by  the  pressure  of  the  atmosphere,  or  by  some  other 
cause. 

He  caused  a  friend  to  repeat  TORRICELLI'S  experiment 
upon  the  top  of  the  mountain  of  Puy-de-Dome,  correctly 
reasoning,  that  if  the  height  of  the  mercurial  column  is  due 
to  atmospheric  pressure  alone,  it  ought  not  to  be  so  great 
on  the  mountain  top  as  at  the  level  of  the  sea.  The  result 
of  the  experiment  showed  that  the  height  of  the  column 
was  less  on  the  top  of  the  mountain  than  at  its  base. 

He  next  reasoned,  that  if  the  tube  were  filled  with  any 
liquid  less  dense  than  mercury,  the  height  of  the  column 
ought  to  be  proportionally  greater.  Consequently,  he  made 
at  Rouen,  in  1646,  the  following  experiment.  He  took  a 
tube,  similar  to  that  of  TORRICELLI,  but  nearly  50  feet  in 
length,  and  after  filling  it  with  wine,  inverted  it  in  a  vessel 
of  the  same  liquid.  PASCAL  observed  that  the  column  fell, 
until  it  was  about  35  feet  high,  when  it  came  to  rest.  In 
this  case,  the  column  was  fourteen  times  as  high  as  when 
mercury  was  used,  and  as  mercury  is  fourteen  times  as  dense 
as  wine,  he  concluded  that  the  sole  cause  of  the  phenomenon 
in  question  was  the  pressure  of  the  atmosphere. 

The  Barometer. 

109.  A  BAROMETER  is  an  instrument  for  measuring  the 
pressure  of  the  air.     If,  to  TORRICELLI'S  tube,  were  fitted  a 


(108.)  Describe  PASCAL'S  experiments  in  detail,  and  his  mode  of  reasoning. 
What  conclusion  is  derived  from  PASCAL'S  experiments?  (109)  What  is  a  Bar 
ometer  ?  What  is  its  principle  ? 


114 


POPULAR   PHYSICS. 


scale  for  measuring  the   exact   altitude    of  the   mercurial 
column,  it  would  be  a  barometer. 

Several  forms  have  been  given  to  the  barometer,  some  of 
which  will  be  described  in  the  following  articles. 

The  Cistern  Barometer. 

11O.  Fig.  76  represents  a 
CISTERN  BAROMETER,  such  as 
is  in  common  use  in  France 
and  in  this  country. 

It  consists  of  a  glass  tube, 
ai,    about    34    inches    long, 
closed  at  the  top  and  open  at 
the  bottom.     This  tube  has  a 
diameter  of  about  four-tenths 
of  an  inch.     It  is  filled  with 
mercury   and   inverted  in   a 
cistern,  A,  which  is  partially 
filled  with  the  same  liquid,  as 
explained  in  Article  107.   The 
mercury  settles  in  the  tube 
till  the  height  of  the  column 
is  about  30  inches  at  the  level 
of  the  sea. 

The  cistern  A,  is  3  or  4 
inches  in  diameter,  and  it  is 
so  adapted  to  the  tube  at,  as 
to  permit  the  air  to  penetrate 
to  the  cistern  at  the  joint  i. 
Only  a  part  of  the  cistern  is 
seen  in  the  figure,  the  remain 
der  being  let  into  the  frame 
which  supports  the  whole  in 
strument.  At  the  top  of  the 


Pic.  76. 


(110.)  Describe  the  Cistern  barometer.    The  tube. 


The  cetera. 


THE    ATMOSPHERE.  115 

frame  is  a  scale,  (7,  having  its  0  point  at  the  level  of  the 
mercury  in  the  cistern  ;  or  on  the  opposite  side,  is  a  scale  on 
which  are  marked  certain  weather  indications. 

A  curved  piece  of  metal  embraces  the  tube  and  carries  an 
index,  which,  as  the  piece  is  raised  or  depressed  to  corres 
pond  to  the  top  of  the  column,  points  out  upon  the  scale  (7, 
the  height  of  the  column.  Two  thermometers,  one  of  mer 
cury  and  one  of  alcohol,  are  also  attached  to  the  frame, 
which  serve  to  show  the  temperature  of  the  instrument  and 
of  the  mercury  which  it  contains. 

The  0  point,  or  beginning  of  the  scale,  is  at  the  surface  of  the 
mercury  in  the  cistern.  When  the  pressure  of  the  air  increases,  a 
portion  of  the  mercury  in  the  cistern  is  forced  up  into  the  tube,  and 
the  0  point  descends  ;  when  the  pressure  diminishes,  the  reverse 
takes  place.  But  inasmuch  as  the  surface  of  the  mercury  in  the 
cistern  is  very  great  in  comparison  with  that  in  the  tube,  this  rise 
and  fall  is,  for  most  purposes,  quite  unimportant.  When  great 
accuracy  is  required,  the  bottom  of  the  cistern  is  made  of  leather,  and 
can.  by  means  of  a  screw,  be  raised  or  depressed  until  the  surface  of 
the  mercury  in  the  cistern  just  grazes  the  point  of  an  ivory  pin  pro 
jecting  from  the  top  of  the  cistern.  This  improvement,  devised  by 
FORTIN,  is  now  in  general  use. 

To  determine  the  height  of  the  barometer,  the  0  point  is 
first  adjusted,  then  the  curved  piece  is  slid  up  or  down  till 
it  coincides  with  the  surface  of  the  mercury  in  the  tube,  and 
the  height  is  then  read  off  on  the  scale  c.  The  height  of 
the  thermometer  should  also  be  noted. 

In  the  instrument  described,  the  scale  c  does  not  extend  through 
out  the  whole  length  of  the  instrument,  because,  in  ordinary  cases, 
only  a  small  part  of  the  scale  is  needed.  When  a  barometer  is  to  be 
used  in  high  altitudes,  the  scale  is  continued  downwards  as  far  as 
necessary. 

Describe  the  scale.  The  -index.  The  thermometers.  Where,  is  the  0  point  of  the 
scale  ?  How  is  the  0  point  regulated  in  accurate  barometers  ?  How  is  the  height 
of  the  barometer  determined  ? 


116 


POPULAR    PHYSICS. 


The  Siphon  Barometer. 

111.  Fig.  77  represents  a  SIPHON 
BAROMETER.  It  consists  of  a  curved 
tube,  «6,  having  two  unequal  branch 
es,  the  shorter  one  acting  as  a  cistern. 
In  the  longer  branch,  there  is  a 
vacuum  above  the  mercury,  but 
the  shorter  one  is  supplied  with  air, 
which  communicates  with  the  external 
atmosphere  through  a  small  opening, 
i.  There  are  two  scales,  one  at  the 
upper  part  of  each  branch,  and  in  front 
of  each  is  a  movable  index  which  may 
be  raised  or  depressed  until  it  comes 
to  the  free  surface  of  the  mercury  in 
each  branch.  By  means  of  these  scales, 
the  difference  of  level  in  the  two 
branches  may  be  measured.  This  dif 
ference  is  the  height  of  the  barometric 
column. 

To  prevent  violent  oscillations  when  the 
instrument  is  moved  from  place  to  place,  the 
two  branches  communicate  through  a  fine, 
almost  capillary,  tube.  This  arrangement 
also  prevents  the  possibility  of  a  bubble  of 
air  penetrating  from  the  shorter  to  the  longer 
branch,  when  the  instrument  is  inclined. 


Ifig.  T7. 


Properties  of  a  good  Barometer. 
112.     The  space  at  the  top  of  the  tube  should  be  per- 

(  111.)  Describe  the  Siphon  barometer.  What  takes  the  place  of  a  cistern?  How 
many  scales  are  needed,  and  how  are  they  arranged  ?  How  is  the  difference  of  level 
determined?  How  are  oscillations  obviated?  (112.)  What  are  the  qualifications 
of  a  good  barometer  ? 


THE   ATMOSPHERE.  117 

fectly  free  from  air  or  moisture,  otherwise  they  would,  by 
their  elastic  force,  prevent  the  mercurial  column  from  rising 
to  its  proper  height. 

The  elastic  force  of  vapor  of  water  is.  as  will  be  shown,  very  con 
siderable,  even  at  ordinary  temperatures.  To  expel  both  air  and 
moisture,  the  mercury  shoujd  be  boiled  in  the  tube  before  the  latter 
is  in-verted  into  the  cistern. 

The  mercury  should  be  pure,  the  bore  of  the  tube  should 
be  sufficiently  large,  and  the  scale  should  be  accurate.  Mer 
cury  may  be  purified  by  distillation. 

Thus  far.  mercury  has  been  preferred  to  all  other  liquids  for  filling 
barometers.  It  is  true,  other  liquids  might  be  used,  but  in  such 
case,  the  tube  would  become  unwieldly  from  its  length.  In  the  case 
of  water,  a  tube  of  about  35  feet  would  be  required.  There  is 
another  objection  to  using  water,  which  arises  from  its  tendency  to 
form  vapor  even  at  ordinary  temperatures.  The  formation  of  vapor 
at  the  top  of  the  tube,  would,  as  we  have  just  seen,  prove  highly 
injurious  to  the  working  of  the  instrument. 

Mean  Height  of  the  Barometer. 

113.  The  height  ot  the  barometer  is  constantly  fluc 
tuating.  The  difference  between  the  greatest  and  least 
heights  observed  at  Paris,  amounts  to  as  much  as  one- 
thirteenth  part  of  the  greatest.  The  fluctuations  become 
greater  as  we  approach  the  poles,  and  less  as  we  approach 
the  equator. 

The  mean  or  average  height  at  any  place  can  be  found  only  from 
a  great  number  of  observations.  If  we  take  hourly  observations  for 
one  day  and  divide  the  sum  of  the  heights  by  24,  the  result  is  called 
the  mean  height  for  that  day.  This  does  not  differ  much  from  the 
height  observed  at  midnight.  If  we  take  the  sum  of  the  mean  daily 
heights  for  a  year,  and  divide  by  365,  the  result  is  the  mean  height 

What  liquid  is  best  for  filling  barometers?  Objections  to  other  liquids? 
(113.)  Where  are  the  fluctuations  of  the  barometer  greatest?  Least?  Amount  at 
Paris  ?  How  is  the  mean  height  for  a  day  determined  ?  For  a  ysar  f 


118  POPULAR     PHYSICS. 

for  that  year.  By  taking  the  sum  of  the  mean  annual  heights  for 
many  years  and  dividing  it  by  the  number  of  years,  the  result  is  the 
mean  height  for  that  place. 

At  the  level  of  the  sea,  the  mean  height  is  not  far  from 
30  inches,  as  already  stated.  4- 

Causes   of  Barometrical  Fluctuations. 

114.  The  cause  of  the  fluctuations  observed  in  the 
barometer,  is  a  change  in  the  weight  of  the  column  of  air 
above  it.      Since  the  weight  of  the  entire  atmosphere  is 
constant,  if  it  become  heavier  at  one  point  on  the  earth's 
surface,   it  must   become   lighter  at  some  other  point ;  a 
fact  which  is  confirmed  by  observations  by  means  of  the 
barometer. 

The  cause  of  the  change  of  weight  in  the  column  of  air 
over  the  barometer,  is  a  change  of  temperature.  When  the 
temperature  at  any  place  is  elevated,  the  air  expands  and 
rises  upward  until  its  lateral  tension  is  greater  than  that  of 
the  surrounding  air,  when  it  flows  away  to  the  neighboring 
regions.  When,  on  the  contrary,  the  temperature  is  dimin 
ished,  the  air  contracts  and  an  additional  quantity  flows  in 
from  the  neighboring  regions. 

The  barometer,  then,  falls  where  there  is  a  dilatation,  and 
rises  where  there  is  a  contraction,  of  the  air. 

The  barometer  serves  as  a  weather-glass.  It  stands  high  in  fair 
weather,  and  low  in  foul  weather.  A  sudden  fall  of  the  barometer 
indicates  an  approaching  storm,  and  a  sudden  rise,  in  general,  indi 
cates  approaching  fair  weather. 

The  Index  Barometer. 

115.  Fig.  78  represents  an  ornamental  form  of  an  INDEX  BAR 
OMETER.     The  manner  in  which  the  index   is   made  to   show  the 

For  any  place  ?  What  is  the  mean  height  at  the  level  of  the  sea  ?  (114)  What 
is  the  cause  of  the  fluctuations  observed  ?  What  is  the  cause  of  the  change  of  weight 
in  the  aerial  column  ?  When  does  the  barometer  rise  ?  Fall  ?  Use  of  the  barometer 
as  a  weather-glaas  f  (115.)  Explain  the  Index  barometer. 


THE    ATMOSPHERE. 


119 


fluctuations  of  the  barometer,  is  shown  in  Fig.  79.  The  index  is 
attached  to  an  axis  which  bears  a  pulley.  Passing  over  this  pulley 
is  a  fine  wire,  at  one  extremity  of  which  is  attached  an  iron  weight, 


Fig.  78.  Fig.  79. 

a.  which  rises  when  the  height  of  the  mercury  diminishes,  and  falls 
when  this  height  increases.  At  the  second  extremity  is  a  counter 
poise,  6,  which  keeps  the  wire  tense,  and  causes  the  wheel  to  turn 
as  the  weights  rise  and  fall. 


120 


POPULAR     PHYSICS. 


The  index  plays  in  front  of  a  dial-plate,  around  which  are  marked 
certain  letters  indicating  the  weather  to  be  expected  when  the  index 
stands  at  any  one  of  them.  The  instrument  shown  in  the  figure  is 
of  French  construction,  and  the  letters  are  the  initials  of  the  French 
names  of  the  different  kinds  of  weather,  as  exhibited  below.  In  the 
annexed  table  is  shown  the  height  of  the  barometer  corresponding  to 
each  indication : 

TABLE. 


HEIGHT  OF 
BAROMETER. 

LETTERS. 

FRENCH. 

ENGLISH. 

28.78  inches. 
29.13       " 
29.48       " 

T. 

G.  P. 
P.  V. 

Tempete. 
Grande  pluie. 
Pluie  ou  vent. 

Tempest. 
Heavy  rain. 
Rain  or  wind. 

29.84       '• 

V. 

Variable. 

Variable. 

30.19       " 
30.54       " 

B. 
B.  F. 

Beau  temps. 
Beau  fixe. 

Fine  weather. 
Settled  weather. 

30.90       c: 

T.  S. 

Tres-sec. 

Drought. 

The  above  table  is  only  given  to  illustrate  the  method  of  employ 
ing  the  instrument.  It  is  evident  that  different  tables  would  be 
required  at  different  places.  But  little  reliance  is  to  be  placed  on 
barometers  of  this  kind,  as  weather  indicators. 

Measure   of  Mountain  Heights  by  the  Barometer. 

116.  One  of  the  most  important  applications  of  the 
barometer,  is  to  the  measurement  of  the  height  of  any  place 
above  the  level  of  the  sea. 

As  we  ascend  above  the  level  of  the  sea.  the  pressure  of  the  air 
diminishes,  and  the  barometer  falls.  Formulas  have  been  deduced, 
by  means  of  which  the  difference  of  level  between  any  two  places 
can  be  found,  when  we  have  the  heights  of  the  mercurial  columns  at 
the  two  places,  together  with  the  temperatures  of  the  air  and  mer 
cury  at  these  places. 

A  detailed  explanation  of  the  method  of  making  the  observations, 


Its  construction  and  use.    (  1 16.)  On  what  principle  is  the  barometer  used  for 
measuring  heights  ? 


THE   ATMOSPHERE. 

and  deducing  the  difference  of  level,  does  not  come  within  the  plan 
of  this  work.  For  information  on  this  subject,  the  reader  is  referred 
to  Mechanics,  Art.  188. 

Height  of  the  Atmosphere. 

117.  The  density  of  the  air  at  the  surface  of  the  earth 
is  about  10,400  times  less  than  that  of  mercury.     Were 
there  no  decrease  in  density  as  we  ascend,  its  height  would 
be  10,400  times  30  inches,  or  26,000  feet ;  that  is,  about  five 
miles.     But  on  account  of  the  rapidly  decreasing  density 
upwards,  the  actual  height  is  very  much  greater.     It  has 
been  estimated  to  be  not  far  from  forty-five  miles  in  height. 

Atmospheric  Pressure  transmitted  in  all  directions. 

118.  Gases,  as  well  as  liquids,  transmit  pressures  in  all 
directions,  from  which  it  results  that  the  pressure  of  the  air 
is  not  only  felt  downwards,  but  laterally  in  all  directions. 
This  is  shown  by  the  Magdebourg  hemispheres,  which  ad 
here  with  equal  force,  whether  the  force  to   draw  them 
asunder  be  exerted  vertically,  laterally,  or  in  any  oblique 
direction. 

The  same  fact  may  be  illustrated  as  follows  :  Let  a  tum 
bler  be  filled  with  water,  and  covered  with  a  sheet  of 
paper ;  then,  holding  the  paper  in  contact  with  the  water, 
let  the  tumbler  be  inverted.  If  the  hand  be  withdrawn,  the 
water  remains  in  the  tumbler,  being  held  there  by  the  pres 
sure  of  the  atmosphere,  directed  upwards,  as  shown  in 
Fig.  80. 

The  wine-taster,  shown  in  Fig.  81,  is  constructed  on  this  prin 
ciple.  It  consists  of  a  tube  open  at  both  ends,  the  lower  opening 
being  quite  small.  The  instrument  is  introduced  into  a  cask  of 


(117.)  Were  the  density  the  same  as  at  the  earth's  surface,  what  would  be  its 
height?  What  is  its  estimated  height?  (118.)  How  are  pressures  transmitted 
through  gasea  ?  How  is  the  principle  illustrated  ?  What  is  the  principle  of  the 
urine-taster  f 

6 


122  POPULAR     PHYSICS. 

wine  through  the  bung-hole,  and  when  it  has  become  filled  to  the 
level  of  the  liquor  in  the  cask.,  the  thumb  is  placed  over  the  upper 
end,  and  the  instrument  is  withdrawn.  A  portion  of  the  wine  is 


Fig.  80.  Fig.  81. 

held  in  the  tube,  being  retained  by  the  atmospheric  pressure,  and  if 
the  tube  be  placed  over  a  tumbler,  and  the  thumb  be  raised,  the 
wine  will  flow  out.  This  is  the  principle  of  the  dropping  tube,  em 
ployed  by  druggists  and  others. 

Pressure  on  the   Human  Body. 

119.  The  pressure  on  each  square  inch  of  the  body  is 
15  Ibs. ;  hence,  on  the  whole  body  the  pressure  is  enormoiiF. 
If  we  take  the  surface  of  the  human  body  equal  to  2000 
square  inches,  which  is  not  far  from  the  average  in  the  case 
of  an  adult,  the  pressure  amounts  to  30,000  pounds,  or 
15  tons. 

If  it  be  asked  why  the  body  is  not  crushed  by  this  enor 
mous  pressure,  the  answer  is,  because  it  is  uniformly  distrib 
uted  over  the  whole  surface,"  and  is  resisted  by  the  elastic 
force  of  air,  and  other  gases,  distributed  through  the  tissues 
of  the  body. 

The  following  experiment  shows  that  the  tissues  of  the  human 


Describe  it  audits  use.     What  is  the  dropping  title?     (119.)  What  is  the 
amount  of  atmospheric  pressure  on  the  human  body  ?    How  is  this  pressure  resisted  ? 


THE     ATMOSPHERE. 


123 


body  contain  air  and  gases,  whose  elasticity  resists  the  atmospheric 
pressure.  Let  the  hand  be  pressed  closely  upon  the  mouth  of  a 
glass  cylinder,  whose  interior  communicates  with  the  air-pump,  as 
shown  in  Fig.  82.  No  inconvenience  will  be  felt.  But  if  the  air 
be  exhausted  from  the  cyl 
inder,  the  flesh  of  the  hand 
will  be  forced  into  the  cyl 
inder  by  the  pressure  from 
without,  which  is  no  longer 
resisted  by  the  pressure  of 
the  air.  The  hand  swells. 
and  the  blood  tends  to  flow 
out  through  the  pores. 

The  question  may  be 
asked,  why,  when  the  hand 
is  placed  upon  a  body,  it  is 
not  retained  there  by  the 
pressure  of  the  atmosphere. 
The  answer  is,  there  is  a 
thin  layer  of  air  between 
the  hand  and  the  body, 
which  exactly  counterbal 
ances  the  effect  of  the  ex 
ternal  pressure.  Were  the 
air  perfectly  excluded  from 
between  the  hand  and  the 
body,  there  would  be  a 
strong  tendency  to  adher 
ence  between  them. 

The  operation  of  cupping,  in  medicine,  depends  upon  the  principle 
just  explained. 


How  shown  that  the  tissues  of  the  body  contain  yases  f    Explain  experiment. 
Principle  of  cupping. 


124:  POPULAR     PHYSICS. 

II.  —  MEASURE       OF      THE       ELASTIC       FORCE       OF       GASES. 

Mariotte's  Law. 

120.  When  a  given  mass  of  any  gas  or  vapor  is  com 
pressed,  so  as  to  occupy  a  smaller  space,  its  elastic  force  is 
increased ;  on  the  contrary,  when  the  volume  is  increased, 
its  elastic  force  is  diminished. 

The  law  of  increase  and  diminution  of  elastic  force  was 
first  made  known  by  MAKIOTTE  ;  hence  it  was  called  by  his 
name.  MARIOTTE'S  law  may  be  enunciated  as  follows : 

The  elastic  force  of  any  given  amount  of  gas,  whose  tem 
perature  remains  the  same,  varies  inversely  as  its  volume. 

As  a  consequence  of  this  law  it  follows  that, 

If  the  temperature  remains  constant,  the  elastic  force 
varies  as  the  density. 

Mariotte's  Tube. 

121.  MAEIOTTE'S  law  may  be  verified  by  means  of  an 
apparatus,  shown  in  Figs.  83  and  84,  called  Mariotte's  Tube. 
This  tube  is  of  glass,  bent  into  the  shape  of  a  letter  J.     The 
short  branch  is  closed,  and  the   long  one  open  at  the  top. 
The  tube  is  attached  to  a  wooden   frame,  provided  with 
suitable  scales  for  measuring  the  heights  of  mercury  and  air 
in  the  two  branches. 

The  instrument  having  been  placed  vertical,  a  sufficient 
quantity  of  mercury  is  poured  into  the  long  branch  to  cut 
off  commimication  between  the  two  branches,  as  shown  in 
Fig.  83.  The  level  of  the  mercury  in  the  two  branches  is 
the  same,  and  this  level  is  at  the  0  point  of  the  two  scales. 
The  air  in  the  short  branch  is  of  the  same  density,  and  has 
the  same  tension  as  that  of  the  external  atmosphere. 

(120.)  What  is  MARIOTTE'S  Law?    Consequence?    (121.)  Describe  Mariotte's 
Tube. 


ELASTIC     FORCE      OF     GASES. 


125 


If  an  additional  quantity  of  mercury  be  poured  into  the 
longer  branch  of  the  tube,  it  will  press  upon  the  air  in  the 
shorter  branch,  and  compress  it.  If  the  difference  of  level 


Fig.  84. 


in  the  two  branches  be  made  equal  to  the  height  of  the 
barometrical  column,  as  shoAvn  in  Fig.  84  (where  the  differ 
ence  is  76  centimetres,  or  29.92  inchcs\  the  air  will  be  com- 
pressed  into  BC,  one  half  of  its  original  bulk. 


How  used  to  verify  the  law  ? 


126  POPULAR     PHYSICS. 

Iii  the  figure,  the  air  in  DC  is  subjected  to  the  pressure  of  two 
atmospheres,  one  from  the  actual  atmosphere,  transmitted  through 
the  mercury,  and  an  equal  pressure  from  the  weight  of  the  mercury, 
AC,  which  is  equal  to  that  of  an  atmosphere. 

If  the  difference  of  height,  AC,  be  made  equal  to  two,  three,  four, 
&c.,  times  that  of  the  barometric  column,  the  air  in  BC  will  be 
reduced  to  one  third,  one  fourth,  one  fifth,  &c.,  of  its  original  bulk. 


Manometers. 

122.  A  MANOMETER  is  an  apparatus  for  measuring  the 
clastic  force  of  a  gas  or  vapor. 

There  are  two  principal  kinds  of  manometers,  the  open 
and  the  dosed  manometer. 


The  Open  Manometer. 

123.  Fig.  85  represents  an  OPEN  MANOMETER,  such  as 
is  often  used  for  measuring  the  pressure  of  steam  in  a  boiler. 

It  consists  of  a  narrow  tube  of  glass  fixed  against  a  verti 
cal  wall,  and  communicating  with  a  cistern  of  mercury,  C. 
A  pipe  leads  from  the  boiler  to  the  cistern,  C,  and  by  means 
of  a  stop-cock,  steam  may  be  admitted  to  the  cistern,  or  cut 
off  at  pleasure. 

When  the  tension  of  the  steam  in  the  boiler  is  just  equal  to  that 
of  the  atmosphere,  the  mercury  stands  at  the  same  level  in  the  tube 
and  cistern.  When  the  tension  of  the  steam  becomes  equal  to  twice 
that  of  the  atmosphere,  the  mercury  is  forced  from  the  cistern  into 
the  pipe,  where  it  rises  till  the  difference  of  level  is  30  inches.  This 
is  marked  2  on  the  tube,  and  when  the  mercury  is  at  this  division,  the 
tension  of  the  steam  is  two  atmospheres.  The  divisions  3,  4,  5,  &c., 
are  placed  at  distances  of  30  inches,  and  when  the  mercury  stands 
at  any  one  of  them,  the  manometer  indicates  a  tension  of  the  corre 
sponding  number  of  atmospheres. 


( 1 22-)  What  is  a  Manometer?    How  many  kinds  are  employed  ?    Whatarethey? 
(  123.)  Describe  the  Open  Manometer.    Explain  its  action. 


ELASTIC    FOKCE     OF     GASES. 


127 


**~ 
!iii    !ii^« 

-J- 


In  the  figure,  the  tension  indicated  is  3£  atmospheres. 

The   Closed  Manometer. 

124.  The  CLOSED  MANOMETER  is  shown  in  Fig.  86,  and 
differs  from  the  one  just  described,  in  having  its  vertical 
tube  closed  at  the  top.  It  is  graduated  on  the  principle 
enunciated  in  MAEIOTTE'S  law. 


(  1 24.)  Describe  the  Closed  Manometer.    How  is  it  graduated  ? 


128 


POPULAR    PHYSICS. 


When  the  pressure  in  the  boiler  is 
one  atmosphere,  the  mercury  in  the 
cistern  and  tube  are  at  the  same  level, 
the  tension  of  the  steam  and  the  elastic 
force  of  the  air  just  balancing  each 
other.  When  the  pressure  becomes 
two,  three,  four,  &c.,  atmospheres,  the 
air  in  the  closed  tube  will  occupy  one 
half,  one  third,  one  fourth,  &c.,  the 
space  it  did  before,  allowance  being 
made  for  the  weight  of  the  mercury 
which  is  forced  up  into  the  tube.  The 
instrument  having  been  graduated,  its 
use  is  evident.  When  it  is  desired  to 
ascertain  the  tension  of  the  steam  in 
the  boiler,  the  cock  is  turned,  and  the 
height  to  which  the  mercury  ascends 
in  the  tube,  indicates  the  tension  in 
atmospheres.  Any  number  of  sub 
divisions  may  be  made  in  either  of  the 
two  manometers  described. 

Besides  these,  there  is  a  metallic 
manometer,  invented  by  M.  BOURDON, 
and  known  as  BOURDON'S  Metallic 


Fig.  SC. 


Manometer.     It  is  not  so  reliable  as  those  described. 


III. APPLICATION  TO   PUMPS   AND   OTHER   MACHINES. 

The  Air-pump. 

125.  An  AIR-PUMP  is  a  machine  for  exhausting  the  air 
from  a  closed  space.  The  air-pump  was  invented  by  OTTO 
VON  GUERICKE,  in  1650. 

A  perspective  view  of  one  of  the  most  common  forms  of  the  air- 
pump  is  given  in  Fig.  87.  The  details  of  its  construction  will  be 
best  studied  from  Figs.  88  and  89  ;  the  former  represents  a  longi- 


Illustrate.     How  is  this  manometer  used? 
When  invented,  and  by  whom  ? 


(125.)  What   is  an   Air-pump? 


PUMPS    AND    OTHER    MACHINES. 


129 


Fig.  87. 


In  all  of  the  figures,  tli 


tudinal,  and  the  latter  a  transverse  section, 
same  letters  indicate  corresponding  parts. 

The  air-pump  consists  of  two   glass   cylinders,   called   barrels,  ir 
which  are  pistons,  P  and  Q.  made  of  leather,  thoroughly  soaked  i 
oil.     The  pistons  arc  attached  to  rods,  and  are  elevated  and  depressed 
by  a  lever.  NM,  Fis.  89,  which  imparts  an  oscillating  motion  to  a 
pinion,  K.     The  teeth  of  this  pinion  engage  with  corresponding 

~~  complete  description  of  the  air-pump.    Barrels.    Pistons     Rode. 

6* 


130 


POPULAK     PHYSICS. 


on  the  inner  sides  of  the  piston  rods,  A  and  B.  The  machine  is  so 
arranged  that  one  rod  ascends  whilst  the  other  descends.  The  cyl 
inders  rest  upon  and  arc  firmly  attached  to  a  platform,  H.  Fig.  88. 
On  the  same  platform,  H.  is  a  column,  I.  which  supports  a  plate,  G. 
Resting  upon  the  plate  G,  is  a  bell  glass.  #,  called  a  receiver.  The 
receiver  communicates  with  both  cylinders  by  a  pipe,  shown  in 
Fig.  88. 


This  pipe  branches  near  the  cylinders,  one  branch  leading  to  each 
cylinder,  as  shown  in  Fig.  89.  The  pipe  communicates  with  the 
cylinders  by  openings,  which  may  be  closed  by  conical  valves,  a  and 
b.  The  valves  a  and  b  are  attached  to  rods  which  pass  through  the 
pistons,  and  fitted  to  slide  with  gentle  friction  as  the  pistons  move 
up  and  down.  In  the  pistons  are  valves,  s  and  £,  which  arc  gently 

Receiver.    Pipe.     Valves.     Valve  rods. 


PUMPS    AND    OTHER    MACHINES. 


131 


pressed  by  spiral  springs  so  as  to  permit  the  condensed  air  to  escape 
and  then  to  close  the  orifices  in  the  valves.-  All  of  the  valves,  a.  3, 
*,  and  £,  open  upwards. 

In  explaining  the  action  of  the  air-pump,  it  will  be  suffi 
cient  to  consider  a  single  barrel,  as  shown  in  Fig.  88.  The 
piston,  P,  being  at  the  bottom  of  the  barrel,  the  valves  a, 
and  t  are  closed.  If  the  piston  be  raised,  the  valve  a  ic 
opened,  whilst  the  valve  t  is  kept  closed  by  the  spiral  spring 
and  the  pressure  of  the  atmosphere.  The  valve  a  is  soon 
arrested  by  its  rod  com 
ing  in  contact  with  the 
top  of  the  barrel,  and  it 
then  remains  open  .during 
the  ascent  of  P.  The  air 
in  the  barrel  above  the 
piston  is  driven  out  at 
the  opening,  r,  and  that 
in  the  receiver  and  pipe 
expands  so  as  to  fill  the 
receiver,  pipe,  and  barrel. 
If  the  piston,  P,  be  de 
pressed,  it  at  once  closes 
the  valve  a,  and  com 
presses  the  air  in  the 
barrel  till  its  elastic  force 
becomes  great  enough  to 
force  open  the  valve  t, 
when  it  escapes  into  the 
atmosphere. 

By  this  double  stroke 

of  the  piston,  P,  a  portion  of  the  air  is  exhausted  from  the 
receiver,  and  if  a  second  double  stroke  be  made,  a  portion 
of  what  remains  may  in  like  manner  be  exhausted,  and  so 
on  until  nearly  a  perfect  vacuum  is  formed  in  the  receiver, 


Fig.  89. 


Describe  the  action  of  the  air-pump  in  detail. 


1 32  POPULAR     PHYSICS. 

72,  or  in  any  other  closed  vessel  attached  to  the  pipe  of  the 
machine. 

What  has  been  said  of  one  barrel,  is  equally  true  of  the 
other  ;  in  fact,  the  instrument,  as  figured,  is  a  double  pump. 


Measure  of  the  Rarefaction  produced. 

126.  In  order  to  measure  the  degree  of  rarefaction  pro 
duced,  a  glass  cylinder,  E,  Fig.  87,  is  connected  with  the 
pipe  by  means  of  an  opening  through  the  column  I.  In  this 
cylinder,  is  a  glass  tube  bent  into  the  form  of  the  letter  U, 
one  branch  being  closed  at  the  top,  and  the  other  open. 
The  tube  has  its  closed  branch  filled  with  mercury,  and  is 
called  a  siphon  gauge. 

The  mercury,  under  ordinary  circumstances,  is  kept  in  the 
closed  branch  by  the  atmospheric  pressure,  but  as  the  air 
becomes  rarefied  in  the  receiver,  the  tension  of  the  air 
becomes  less  and  less,  and  finally  the  mercury  falls  in  the 
closed  branch  and  rises  in  the  open  one.  The  difference  of 
level  between  the  mercury  in  the  two  branches,  is  due  to  tlr, 
tension  of  the  rarefied  air,  and  if  this  difference  be  deter 
mined  by  means  of  a  proper  scale  attached  to  the  gauge 
the  tension  can  be  found.  Thus,  if  the  difference  of  level  is 
reduced  to  one  inch,  the  tension  of  the  air  in  the  receiver 
will  be  only  one  thirtieth  part  of  the  cension  of  the  external 
atmosphere. 

.Experiments  with  the  Air-pump. 

ItSY.  We  have  already  described  several  experiments  requiring 
the  employment  of  the  air-pump,  such  as  the  shower  of  mercury. 
Fig.  1 :  the  fall  of  bodies  in  a  vacuum,  Fig.  2 ;  the  bladder  in  a 
vacuum,  Fig.  70  ;  the  bursting  membrane,  Fig.  72  ;  and  finally,  the 
hemispheres  of  Magdebourg,  Fig.  73. 

( 126.)  How  may  the  degree  of  rarefaction  be  measured?    What  is  the  siphon 
gauge  ?    Explain  its  action  and  use. 


PUMPS     AXD     OTIIER     MACHINES. 


133 


The  machine  may  be  used  to  show  that  the  air  is  necessary  to  the 
support  of  combustion  and  ani 
mal  life.  If  a  lighted  taper  be 
placed  under  the  receiver,  and 
the  air  exhausted,  the  light  will 
grow  dim,  and  finally  will  go 
out  entirely.  If  an  animal  or 
bird  be  placed  under  the  re 
ceiver,  and  the  air  exhausted,  it 
will  struggle  and  soon  die.  This 
experiment  is  shown  in  Fig.  90. 

Animals  and  birds  die  as 
soon  as  they  are  placed  in  a 
vacuum  •  reptiles  support  life 
longer  when  deprived  of  air .  ?& 
to  certain  insects,  they  live  for 
many  days  under  an  exhausted 
receiver.  They  are  enabled  to 
live  on  the  small  supply  of  air 
•uhich  remains  in  the  receiver, 
after  as  much  of  it  as  possible 
is  extracted.  Fig  9a 


Preservation  of  Food  in  a  Vacuum. 

128.  It  has  been  discovered  that  articles  of  food  which  would 
soon  perish  if  exposed  to  the  air.  may  be  preserved  fresh  for  a  long 
time  if  kept  in  a  vacuum. 

If  fruits,  vegetables,  and  the  like,  be  placed  in  a  bottle  with 
water,  and  then  heated  gradually  till  ebullition  takes  place,  all  of  the 
air  will  be  driven  out,  being  replaced  by  steam.  If  the  bottle  is 
corked  and  sealed  in  this  condition,  the  fruit  will  remain  fresh  for 
years.  On  this  principle,  vast  quantities  of  meat,  fruit,  vegetables, 
and  the  like,  are  prepared  for  naval  and  other  purposes.  Instead  of 
bottles,  tin  canisters  may  be  employed,  which,  after  expelling  the 
air,  are  hermetically  sealed  by  soldering. 


(  127.)  ITow  is  it  shown  that  air  is  necessary  to  combustion  and  animal  life? 
What  animals  support  life  longest  in  a, vacuum?  (128.)  How  are  articles  of 
food  preserved  in  vacua  ?  What  applications  are  made  of  this  principle  ? 


134: 


POPULAIi     PHYSICS. 


The  Condenser. 


129.  A  CONDENSER  is  a  machine  for  condensing  air,  by 
forcing  large  quantities  into  a  small  space. 

Such  a  machine  is  represented  in  i^ig.  91.  It  is  similar  to 
the  air-pump  in  its  general  construction,  but  differs  in  somo 
of  its  details.  The  receiver  is  of  very  thick  glass,  and  is 


Fig.  91. 


confined  upon  the  plate  by  a  second  plate  at  the  top,  which 
is  connected  with  the  bottom  plate  by  four  brass  rods  with 
suitable  screws  and  nuts.  To  prevent  danger  in  case  of 
rupture,  the  glass  receiver  is  surrounded  by  a  netting  of 
strong  wire.  The  four  valves  open  in  a  direction  contrary 
to  that  of  the  valves  in  the  air-pump,  so  that  air  is  forced 

(  129  )  What  is  a  Condenser  ?    Difference  between  it  and  the  air-pump  ?    Ho\v  is 
the  receiver  guarded? 


PUMPS     AND     OTHER     MACHINES.  135 

into  the  receiver  at  every  double  stroke,  instead  of  being 
exhausted,  as  in  that  instrument.  Finally,  a  closed  manom 
eter,  m,  is  employed  to  indicate  the  tension  of  the  com 
pressed  air.  The  machine  is  worked  in  the  same  way  as  the 
air-pump. 

A  taper  burns  more  freely  in  compressed  air  than  in  the  air  under 
the  ordinary  pressure.  Animals  placed  in  compressed  air  do  not 
experience  any  extraordinary  inconvenience.  In  many  submarine 
operations,  it  becomes  necessary  for  men  to  work  in  an  atmosphere 
of  compressed  air,  and  it  has  been  found  that  no  other  inconvenience 
is  felt  under  a  pressure  of  three  atmospheres,  than  a  painful  sense  of 
compression  in  the  ears.  This  feeling  takes  place  only  at  the 
beginning  and  end  of  the  operations,  disappearing  when  an  equilib 
rium  is  established  between  ihe  tension  of  the  air  in  the  internal  ear 
and  that  without. 

Artificial   Fountains. 

130.  An  ARTIFICIAL  FOUNTAIN,  is  a  machine  by  means 
of  which  water  is  forced  upward  in  the  form  of  a  jet  by  the 
tension  of  compressed  air.     The  most  interesting  instrument 
of  this  class,  is  that  known  as  HERO'S  fountain,  so  named 
from  its  inventor,  HERO,  of  Alexandria,  born  120  B.  c. 

Hero's   Fountain. 

131.  An  ornamental  form  of  HERO'S  FOUNTAIN  is  shown 
in  Fig.  92.     It  consists  of  two  globes  of  glass,  connected  by 
two  metallic  tubes.     The  upper  globe  is  surmounted  by  a 
brass  basin,  connected  with  the  globes  by  tubes,  as  shown 
in  the  figure. 

To  use  the  instrument,  the  tube  which  forms  the  jet  is 
withdrawn,  and  through  the  opening  thus  made,  the  upper 
globe  is  nearly  filled  with  water,  the  lower  one  containing 
air  only.  The  jet  tube  is  then  replaced,  and  some  water 
is  poured  into  the  basin. 

How  is  the  degree  of  condensation  measitred  ?  What  effect  has  condensed  air  on 
combination?  On  animal  life?  On  divers?  (  1  30.)  What  is  an  Artificial  Foun 
tain  ?  (131.)  Describe  HERO'S  Fountain.  How  is  it  prepared  for  use  ? 


136 


P'JPULAK     PHYSICS. 


The  water  in  the  basin,  acting  by  its  weight,  flows  into 
the  lower  globe,  through  the  tube  shown  on  the  left  of  the 
figure,  as  indicated  by  the  arrow  head.  This  flow  of  water 
into  the  lower  globe  forces  out  a  part  of  the  air  in  it,  which, 
ascending  by  the  tube  shown  on  the  right  of  the  figure, 
accumulates  in  the  upper  globe.  The  pressure  of  the  air 
in  the  upper  globe,  acting  upon  the  water  in  that  part  of 

Explain  its  action. 


PUMPS     AND     OTIIEK    MACHINES. 


13T 


the  instrument,  forces  a  part  of  it  up  through  the  jet  tube, 
giving  rise  to  a  jet  of  water,  which  may  be  made  to  play  for 
several  hours  without  re-filling  the  instrument. 


Intermittent  Fountain. 


132.  An  INTERMITTENT  FOUNTAIN  is  one  in  which  the 
flow  is  intermittent,  that  is,  in  which  the  flow  takes  place  at 
regular  intervals.  Such  fountains  exist  in  nature.  Fig.  93 
represents  an  artificial  fountain  of  this  character. 


( 1 32.)  What  is  an  Intermittent  Fountain  ? 


138  POPULAR     PHYSICS. 

It  consists  of  a  glass  globe,  #,  closed  above  by  a  glas.3 
stopper,  and  having  two  small  tubes  below,  through  which 
water  can  flow  without  interruption.  The  globe  «,  is  sup 
ported  by  a  hollow  glass  stem,  J,  which,  rising  from  a  me 
tallic  basin,  enters  the  globe  and  reaches  nearly  to  the  top 
of  it.  Around  the  bottom  of  the  tube  e?,  are  small  holes,  c, 
through  Avhich  air  can  enter  it,  and  thus  reach  the  upper 
part  of  the  globe  a.  A  small  spout,  m,  serves  to  draw  off 
the  water  from  the  basin. 

To  use  the  instrument,  the  globe  «,  and  the  metallic 
basin,  are  nearly  filled  with  water.  So  long  as  the  holes,  c, 
are  covered  with  water,  no  flow  will  take  place  from  the 
globe  a,  but  as  soon  as  the  basin  is  emptied  by  the  spout 
m,  so  as  to  expose  the  holes,  c,  the  air  enters  the  tube  6?, 
and  reaching  the  globe  #,  the  flow  from  the  two  tubes  com 
mences.  The  flow  will  continue  until  the  holes,  <?,  are  again 
submerged,  when  it  will  cease,  and  so  on  as  long  as  any 
water  remains  in  the  globe. 

Of  course  the  capacity  of  the  two  tubes,  attached  to  the 
globe  a,  must  be  greater  than  that  of  the  spout  m. 

The  Atmospheric  Inkstand. 

133.  An  inkstand  has  been  devised  in  accordance  with 
the  principles  of  atmospheric  pressure,  which,  whilst  pre 
serving  the  ink  from  evaporation,  is  extremely  simple  in  its 
construction. 

The  inkstand,  partially  filled  with  ink,  is  represented  in 
Fig.  94.  The  body  of  the  inkstand  is  air-tight.  Near  the 
bottom  is  a  tube  for  supplying  the  ink  as  wanted,  and  also 
for  filling  the  inkstand  when  'necessary.  The  inkstand  is 
filled  by  turning  it  until  the  tube  is  at  the  top,  when  the 


Describe  the  artificial  one  shown  in  Fig.  93.    Explain  its  action     (133.)  Explain 
the  construction  and  use  of  the  Atmospheric  Inkstand. 


PUMPS     AND     OTHER     MACHINES. 


139 


ink  can  be  poured  in  through 
the  tube.  The  pressure  of 
the  atmosphere  prevents  the 
ink  from  flowing  out.  When 
the  ink  has  been  used  till  its 
level  falls  below  o,  where  the 
tube  joins  the  main  body  of 
the  inkstand,  a  bubble  of  air 
enters,  and  rising  to  the  top, 
acts  by  its  pressure  to  fill  the 
tube  again,  and  so  on  until  the 
ink  is  exhausted. 


Fin.  94. 


Water  Pumps. 

134.  A  WATER  PUMP  is  a  machine  for  raising  water 
from  a  lower  to  a  higher  level,  generally  by  the  aid  of 
atmospheric  pressure.      Three   separate  principles  are  em 
ployed  in  working  pumps :  the  sucking,  the  lifting,  and  the 
forcing  principles.     Pumps  are  often  named  according  as 
one  or  more  of  these  principles  are  employed. 

The   Sucking   and    Lifting  Pump. 

135.  A  SUCKING  AND  LIFTING  PUMP  is  represented  in 
Fig.  95,  in  which  a  portion  of  the  barrel  is  removed,  to  show 
more  clearly  the  relative  position  of  the  parts. 

It  consists  of  a  cylinder,  usually  of  cast  iron,  called  the 
barrel  of  the  pump.  The  barrel  communicates  with  a  re 
servoir  by  a  narrow  pipe,  called  the  sucking  pipe,  a  part 
of  which  is  shown  in  the  figure.  At  the  top  of  the  sucking 
pipe  is  a  valve  opening  upwards,  called  the  sleeping  valve. 
Within  the  barrel  is  a  disk  of  metal  or  wood,  packed  with 
leather,  called  the  piston.  The  piston  is  attached  to  a  rod, 
£,  called  the  piston  rod,  and  is  moved  up  and  down  through 

(134.)  What  is  a  Water  Pump  ?  How  many  principles  may  be  employed  ?  What 
are  they?  How  are  pumps  named?  (135.)  Describe  the  Sucking  and  Lifting 
Pump.  Its  barrel.  Sucking  pipe.  Sleeping  valve.  Piston. 


POPULAR    PHYSICS. 


a  certain  space,  called  the  play  of  the  piston,  by  a  lever,  R, 
called  the  pump-handle.  To  cause  the  rod  to  work  verti 
cally,  it  is  connected  with  the  handle  by  a  forked  piece, 


which  is  united  to  the  piston  rod  by  a  hinge  joint.  This 
arrangement  permits  the  rod,  £,  to  glide  up  and  down 
through  a  guide,  as  shown  in  the  figure.  Finally,  the  piston 


Play  of  the  piston.    Piston  rod.    Guide. 


PUMPS     AND     OTHER     MACHINES. 

itself  is  pierced  in  its  centre,  and  carries  a  second  valve, 
also  opening  upward,  called  the  piston  valve. 

In  explaining  the  action  of  this  pump,  we  refer  to  Figs. 
96,  97,  and  98,  which  represent  sections  of  the  pump  in 
different  states  of  action.  In  all  of  the  figures,  a  is  the 
sleeping  valve,  c  the  piston  valve,  and  B  the  sucking  pipe. 


Fig.  96. 


Fig.  97. 


Fig.  98. 


Suppose  the  piston  to  be  at  the  lowest  point  of  its  play ; 
there  will  then  be  an  equilibrium  between  the  pressure  of 
the  air  within  the  pump  and  that  without.  When  the 
piston  is  raised  to  the  highest  point  of  its  play,  the  air  be 
neath  it  is  rarefied,  and  its  tension  diminished ;  the  tension 
of  the  air  in  the  sucking  pipe  then  forces  up  the  sleeping- 
valve,  and  a  portion  of  it  escapes  into  the  barrel.  The  ten 
sion  of  the  air  in  the  sucking  pipe  being  less  than  that  of 


Piston  valve.    Explain  the  action  of  this  pump. 


POPULAR     PHYSICS. 

the  external  atmosphere,  a  quantity  of  water  rises  in  the 
pipe,  to  restore  the  equilibrium.  The  water  continues  to 
rise  till  its  weight,  increased  by  the  tension  of  the  air 
in  the  pump,  is  just  equal  to  the  tension  of  the  external  air. 
When  the  equilibrium  is  restored,  the  sleeping  valve  closes 
by  its  own  weight. 

Now,  if  the  piston  be  depressed,  the  air  in  the  barrel  is 
condensed,  forces  open  the  piston  valve,  and  a  portion 
escapes  into  the  external  atmosphere.  If  the  piston  be 
raised  again,  an  additional  quantity  of  water  will  be  forced 
into  the  pump,  and  after  one  or  two  strokes  of  the  piston, 
it  will  begin  to  flow  into  the  barrel,  as  shown  in  Fig.  96. 

When  the  water  rises  above  the  lowest  limit  of  the  play 
of  the  piston,  the  latter  in  its  descent  will  act  to  compress 
the  water  in  the  barrel.  This  pressure  forces  open  the 
piston  valve,  and  a  portion  of  the  water  passes  above  the 
piston,  as  shown  in  Fig.  97.  By  continuing  to  elevate  and 
depress  the  piston,  the  water  will  be  raised  higher  and 
higher  in  the  pump,  till  at  length  it  will  flow  from  the  spout, 
as  shown  hi  Fig.  98. 

As  the  water  is  raised  in  the  pump  by  atmospheric  pressure,  it  is 
necessary  that  the  lowest  limit  of  the  play  of  the  piston  should  not 
be  more  than  34  feet  above  the  surface  of  the  water  in  the  reservoir, 
even  at  the  level  of  the  sea.  To  provide  against  barometric  fluctua 
tions  and  other  contingencies,  it  is  usual  to  make  this  distance  con 
siderably  less  than  34  feet. 

The  Forcing  Pump. 

136.  In  the  FORCING  PUMP,  the  sucking  pipe  may  be 
dispensed  with,  and  the  barrel  plunged  directly  into  the 
reservoir,  as  shown  in  Figs.  99  and  100,  or  a  sucking 
pipe  may  be  employed,  as  will  be  explained  hereafter.  We 


What  is  the  lowest  limit  of  the  play  of  the  piston  ?    ( 1 3  6. '  What  two  forms  may 
be  given  to  the  Forcing  Pump  ? 


PUMPS  AND  OTHER  MACHINES. 


143 


shall  first  consider  the  case  in  which  the  sucking  pipe  is 
omitted. 


Fig.  99. 


Fig.  100. 


In  this  case  the  piston  is  solid,  and  a  lateral  pipe,  U, 
called  the  delivery  pipe,  is  introduced  below  the  level  of 
the  lowest  position  of  the  piston.  There  are  two  valves, 
both  fixed,  the  sleeping  valve  #,  as  in  the  sucking  pump, 
and  a  valve  c,  opening  into  the  delivery  pipe. 

When  the  piston  is  raised  to  its  highest  position,  as  shown 
in  Fig.  99,  the  pressure  of  the  atmosphere  on  the  water  in 
the  reservoir  forces  open  the  sleeping  valve,  and  the  barrel 
is  filled  with  water  up  to  the  bottom  of  the  piston,  when 
the  sleeping  valve  closes  by  its  own  weight.  On  depressing 
the  piston,  the  valve  c,  is  forced  open,  and  a  portion  of  the 
water  in  the  barrel  is  forced  into  the  delivery  pipe.  When 


Describe  the  piston.    The  delivery  pipe.    Explain  the  action  of  the  forcing  pump 
in  detail. 


144  POPULAR     PHYSICS. 

the  piston  reaches  its  lowest  position,  the  weight  of  the 
water  in  the  delivery  pipe  closes  the  valve  c,  and  prevents 
the  water  in  the  delivery  pipe  from  returning  into  the 
barrel. 

By  continually  raising  and  depressing  the  piston,  addi 
tional  quantities  of  water  are  forced  into  the  delivery  pipe, 
which  finally  escape  from  the  spout  at  the  top  of  the 
delivery  pipe,  as  shown  in  Fig.  100. 

To  regulate  the  flow  of  the  water  through  the  delivery  pipe,  and 
to  facilitate  the  working  of  the  pump,  an  air-vessel  is  generally  in 
troduced,  as  will  be  explained  in  the  next  article.  Sometimes  the 
working  is  rendered  uniform  by  combining  two  forcing  pumps  in 
such  a  manner,  that  the  piston  of  the  one  ascends,  whilst  that  of  the 
other  descends.  This  combination  is  also  explained  in  the  next 
article. 

The  oil  in  a  carcel-lamp  is  forced  up  into  the  wick  by  a  double 
forcing  pump,  moved  by  clock-work. 

The  Fire   Engine. 

137.  A  FIRE  EXGIXE  is  a  double  forcing  pump,  having 
its  delivery  pipe  composed  of  leather  or  other  flexible 
material.  It  is  used,  as  its  name  implies,  for  extinguishing 
fires. 

Fig.  101  shows  a  section  of  the  essential  parts  of  a  fire 
engine.  In  this  figure,  PQ  is  the  lever  to  which  are  at 
tached  the  piston  rods,  that  move  the  pistons  m  and  n ;  Jl 
is  an  air-vessel  with  two  valves,  one  admitting  water  from 
each  barrel ;  Z  is  the  entrance  to  the  hose  or  delivery 
pipe ;  M  and  N  are  rods  sustaining  the  framework  of  the 
machine. 

The  two  barrels  are  plunged  into  a  reservoir  which  is 
kept  supplied  with  water.  This  water  flows  into  a  space 

How  is  the  flow  regulated?  How  is  the  working  rendered  unifor"n?  How  is 
the  oil  raised  in  a  carcel-lamp  f  (137.)  What  is  a  Fire  Engine?  Describe  it  in 
detail. 


PUMPS     AND     OTHER     MACHINES. 


145 


beneath  the  barrels  through  holes  represented  on  the  right 
and  left  of  the  figure,  and  from  thence  is  forced  into  the 
air-vessel  in  a  manner  entirely  similar  tc  that  explained  in 


Fig.  101. 


the  last  article.  When  the  water  is  forced  into  the  air- 
vessel  72,  the  air  is  at  first  compressed,  after  which  it  acts 
by  its  tension  to  force  a  continuous  current  through  the 
hose. 

The  lever  is  provided  with  long  handles  at  right  angles  to  its 
length,  so  that  it  may  be  worked  by  several  men  acting  together. 

Within  a  few  years  many  improvements  have  been  introduced 
into  the  fire  engine,  one  of  the  most  important  being  the  application 
of  steam  as  a  motor. 


How  is  it  supplied  with  water  ?    ITow  is  it  maneuvered  ? 

7 


146 


POPULAR    PHYSICS. 


102. 


The  Sucking  and  Forcing  Pump. 

138.  This  differs  from  the  simple  forcing  pump  described 
in  Art.  136,  in  having  a  sucking  pipe  and  an  air-vessel. 
It  consists  of  a  barrel,  A,  a  sucking 
pipe,  B,  a  sleeping  valve,  G,  and  a 
solid  piston,  C\  worked  by  a  lever,  E, 
and  piston  rod,  D.  A  pipe  leads  from 
the  bottom  of  the  barrel,  through  a 
sleeping  valve,  F,  into  an  air-vessel, 
K.  The  delivery  pipe,  H,  enters  the 
air-chamber  at  its  top  and  extends 
nearly  to  the  bottom. 

To  explain  the  action  of  the  pump, 
suppose  it  empty  and  the  piston  at 
its  lowest  position  ;  when  it  is  raised 
to  its  highest  position,  the  air  in 
the  barrel  is  rarefied,  the  tension  of  the  air  in  the  sucking 
pipe  forces  open  the  valve,  G,  and  a  portion  of  it  escapes 
into  the  barrel ;  the  water  is  then  forced  up  the  suck 
ing  pipe  by  the  tension  of  the  external  air  acting  on 
the  surface  of  the  water  in  the  reservoir  until  an  equilib 
rium  is  produced,  when  the  valve,  G,  closes  by  its  own 
weight.  If  the  piston  be  again  depressed  to  its  lowest  limit, 
the  air  in  the  barrel  is  condensed  until  its  tension  exceeds 
that  of  the  external  air,  when  it  forces  open  the  valve,  F, 
and  a  portion  escapes  into  the  air-vessel.  After  a  few 
double  strokes  of  the  piston  the  water  rises  through  the 
valve,  G,  and  the  action  becomes  the  same  as  in  the  pump 
described  in  Art.  136 ;  with  the  exception  of  the  air-ves 
sel,  which  serves  to  keep  up  a  continuous  stream  through 
the  delivery  pipe.  The  piston  ought  not  to  be  more  than 
34  feet  above  the  reservoir.  The  spout,  P,  may  be  at  any 
height  above  K. 


(138.)  Describe  the  sucking  and  forcing  pump  and  its  mode  of  action. 


PUMPS     AND     OTHER     MACHINES. 


147 


The   Siphon. 

139.  The  SIPHON  is  a  bent  tube,  by  means  of  which  a 
liquid  may  be  transferred  from  one  reservoir  to  another,  over 
an  intermediate  elevation.  The  siphon  may  be  used  with 
advantage  when  it  is  required  to  draw  off  the  upper  portion 
of  a  liquid  without  disturbing  the  lower  portion.  This 
operation  is  called  decanting. 


Y 


Fis  103 


The  siphon  consists  of  two  branches  of  unequal  lengths,  as 
shown  in  Fig.  103.  The  shorter  one  is  plunged  into  the 
liquid  to  be  decanted,  and  the  flow  takes  place  from  the 
longer  one. 

To  use  the  siphon,  it  must  first  be  filled  with  the  liquid.  This 
operation  may  be  effected  by  applying  the  mouth  to  the  outer  end  of 


(189.)    What  is  a  Siphon?    When  may  it  be  used  with  advantage?     What  is 
decanting?    Explain  the  operation.     How  ix  the  siphon  prepared  fas  vsf.t 


148  POPULAR     PHYSICS. 

the  siphon,  and  exhausting  the  air  by  suction,  or  it  may  be  inverted 
and  filled  by  pouring  in  the  liquid,  and  stopping  both  ends,  after 
which  it  is  again  inverted,  care  being  taken  to  open  both  ends  at  the 
same  instant.  Sometimes  a  sucking  pipe  is  employed  to  exhaust  the 
air  and  fill  the  siphon. 

When  the  flow  commences,  it  will  continue  until  the  liquid  in  the 
first  reservoir  falls  below  the  level  of  the  end  of  the  siphon. 

To  understand  the  action  of  the  siphon,  we  must  consider 
the  forces  called  into  play.  The  water  is  urged  from  d 
towards  £,  by  the  pressure  of  the  atmosphere  on  the  fluid  in 
the  reservoir,  together  with  the  weight  of  the  water  in  the 
outer  branch  of  the  siphon  ;  that  is,  by  the  weight  of  a 
column  of  water  whose  height  is  ab.  This  motion  is  re 
tarded  by  the  pressure  of  the  atmosphere  at  #,  together  with 
the  weight  of  the  fluid  in  the  inner  branch  ;  that  is,  by  the 
weight  of  a  column  whose  height  is  cd.  The  difference  of 
these  forces  is  the  weight  of  a  column  of  the  liquid  whose 
height  is  the  excess  of  ab  over  cd,  and  it  is  by  the  action  of 
this  force  that  the  flow  is  kept  up.  The  greater  this  differ 
ence  the  more  rapid  will  be  the  flow,  and  the  less  this 
difference  the  slower  the  liquid  will  escape.  When  this 
difference  becomes  zero,  the  flow  ceases  altogether. 

The  siphon  is  used  for  conveying  water  over  hills,  but  for  this 
purpose  the  highest  point  of  the  tube  should  riot  be  more  than  thirty 
feet  above  the  level  of  the  water  in  the  reservoir,  this  being  about 
the  height  at  which  the  atmospheric  pressure  will  sustain  a  column 
of  water. 

If  a  siphon  be  mounted  on  a  piece  of  cork,  so  as  to  sink  as  the  level 
of  the  fluid  falls,  the  flow  will  be  constant.  Such  a  siphon  is  called 
a  siphon  of  constant  flow. 


lino  long  will  the  flow  continue  t  Explain  the  principle  and  action  of  the  siphon 
in  detail.  How  high  can  water  be  raised  by  a  siphon  t  Describe  a  siphon  of  con* 
stantflow. 


BALLOONING. 


149 


!  y  .  —  APPLICATION       TO       BALLOONING. 

Buoyant  Effort  of  the  Atmosphere. 

14O.     It  has  been  shown  that  a  body  plunged  into  a 
liquid  is  buoyed  up  by  a  force  equal  to  the  weight  of  the 
displaced  liquid.     That  a  similar  eifect  is  produced  upon  a 
body  in   the  atmosphere, 
may  be  shown  by  means 
of  an  instrument  called  a 
baroscope,  which  is  repre 
sented  in  Fig.  104. 

The  BAROSCOPE  consists 
of  a  beam  like  that  of  a 
balance,  from  one  extrem 
ity  of  which  is  suspended 
a  hollow  sphere  of  copper, 
and  from  the  other  ex 
tremity  a  solid  sphere  of 
lead.  These  are  made  to 
balance  each  other  in  the 
atmosphere. 

If  the  instrument  be 
placed  under  the  receiver 
of  an  air-pump  and  the 
air  exhausted,  the  copper 
sphere  will  descend.  This 
shows  that  in  the  air  it 
was  buoyed  up  by  a  force 
greater  than  that  exerted 
upon  the  leaden  sphere. 

If,  now,  the  leaden  sphere  be  increased  by  a  weight  equal 
to  that  of  a  volume  of  air  equal  to  the  bulk  of  the  copper 


Fig.  104. 


( 14O  )  What  instrument  is  used  to  show  the  buoyant  effort  of  the  air?    Describe 
the  Baroscope.    Explain  its  use. 


150  POPULAR     PHYSICS. 

sphere  diminished  by  that  of  the  leaden  sphere,  it  will  be 
found,  after  the  air  is  exhausted,  that  the  balance  is  in 
equilibrium.  This  shows  that  the  buoyant  effort  is  equal  to 
the  weight  of  air  displaced.  Hence  we  have  the  follow 
ing  principle,  entirely  analogous  to  the  principle  of 
ARCHIMEDES: 

When  a  body  is  plunged  into  a  gas,  it  is  buoyed  up  by 
a  force  equal  to  the  weight  of  the  displaced  gas. 

If  the  buoyant  effort  is  greater  than  the  weight  of  the  body,  the 
latter  will  rise  :  if  it  is  less,  the  body  will  fall  ;  if  the  two  are  equal, 
the  body  will  float  in  the  atmosphere  without  either  rising  or  falling. 

Smoke,  for  example,  rises,  because  it  is  lighter  than  the  air  which 
it  displaces.  It  continues  to  rise  until  it  reaches  a  stratum  of  air 
where  its  weight  is  just  equal  to  that  of  the  displaced  air,  when  it 
will  come  to  rest  and  remain  suspended.  A  soap-bubble  filled  with 
warm  air  floats  for  a  considerable  time  in  the  atmosphere,  being 
nearly  of  the  same  weight  as  the  displaced  air. 

The  Balloon. 

141.  A  BALLOON  is  a  spherical  envelope  filled  with  some 
gas  lighter  than  the  air. 

Balloons  are  of  very  different  sizes,  and  are  filled  with  gases  of  very 
different  specific  gravities,  and  consequently  capable  of  raising  very 
different  weights  in  ascending  to  the  upper  regions  of  the  atmosphere. 

The  first  balloon  was  constructed  by  STEPHEN  and  JOSEPH  MONT- 
GOLFIER.  two  brothers,  in  1783.  It  was  made  of  linen,  lined  with 
paper.  It  was  about  forty  feet  in  diameter,  and  weighed  560  Ibs. 
It  was  filled  with  heated  air  and  smoke,  furnished  by  burning  wet 
straw,  paper,  and  t'io  like,  under  the  balloon,  the  lower  part  of  which 
was  Lft  open  to  receive  it.  The  balloon  rose  to  a  height  of  more 
than  a  mile,  but  it  soon  became  cooled  in  the  upper  regions  of  the 
air  and  fell  to  the  earth. 


Give  the  law  of  buoyancy.  Whemcill  a  body  rise  in  the  atmosphere?  When 
fall  t  When  remain  neutral  ?  Examples.  (141.)  "What  is  a  Balloon  ?  Give  an 
account  of  the  early  history  of  ballooning. 


BALLOONING.  151 

In  the  following  August,  two  brothers,  named  ROBERT,  constructed 
a  balloon  of  silk  saturated  with  india-rubber,  and  filled  it  with 
hydrogen  gas.  The  ascensional  power  of  this  balloon  was  very 
great,  and  being  set  loose  in  Paris,  it  rose  with  great  rapidity,  and  at 
the  end  of  four  minutes  had  reached  a  height  of  nearly  a  thousand 
yards,  when  it  was  lost  sight  of  by  entering  a  cloud.  It  descended 
fifteen  miles  from  Paris,  to  the  astonishment  of  the  people  who  saw  it;, 

Manner  of  filling  a  Balloon   and  making  an  ascent. 

142.  Balloons  may  be  filled  either  with  hydrogen  or  with  illu 
minating  gas,  which  is  a  compound  of  carbon  and  hydrogen.  On 
account  of  the  readiness  with  which  the  latter  gas  can  be  obtained, 
together  with  its  cheapness,  it  is  generally  employed.  The  envelope 
is  made  of  silk,  rendered  air-tight  by  some  kind  of  varnish,  and  is 
strengthened  by  a  network  of  cords.  This  network  also  serves  to 
sustain  a  wicker  basket,  or  car,  in  which  the  aeronaut  is  seated. 

Fig.  105  represents  the  method  of  filling  a  balloon,  and  preparing 
it  for  an  ascension.  Two  masts  are  erected  at  a  suitable  distance 
from  each  other,  at  the  tops  of  which  are  pulleys.  A  rope  passing 
through  a  loop  at  the  top  of  the  balloon,  also  passes  over  the  pulleys, 
and  serves  to  raise  the  balloon  during  the  process  of  filling. 

When  the  process  of  filling  commences,  the  balloon  is  raised  till 
it  is  three  or  four  feet  above  the  ground,  when  the  gas  is  introduced 
by  means  of  a  pipe  or  hose  which  connects  with  a  gasometer.  As 
the  balloon  fills  with  gas  it  is  held  down  by  ropes,  and  when  com 
pletely  filled,  the  opening  is  closed,  and  the  car  attached.  Care 
should  be  taken  not  to  fill  the  balloon  completely,  as  the  gas  expands 
in  rising,  and  unless  an  allowance  is  made  for  this  increase  of 
volume,  the  balloon  might  be  ruptured. 

To  regulate  the  ascensional  power,  the  car  is  ballasted  by  sand, 
contained  in  small  bags.  Everything  being  ready,  the  ropes  are 
detached,  and  the  balloon  ascends  with  greater  or  less  velocity, 
according  to  the  ascensional  force,  that  is,  the  excess  of  the  buoyant 
effort  over  the  weight  of  the  entire  balloon  and  its  cargo. 

When  the  aeronaut  finds  that  he  does  not  ascend  fast  enough,  he 
increases  the  ascensional  force  by  emptying  one  or  more  of  the  sand 

(142.)  With  what  are  balloon?  filled  f  Explain  the  method  of  filling  a  balloon. 
How  is  the  ascensional  power  regulated  t 


152 


POPULAR    PHYSICS. 


bags.  In  like  manner,  in  descending,  if  the  velocity  is  too  great,  or 
if  the  balloon  tends  to  fall  in  a  dangerous  place,  the  weight  of  the 
balloon  is  diminished  by  emptying  some  of  the  sand  bags. 


Fig.  105 

To  render  the  descent  less  difficult,  the  aeronaut  is  provided  with 
AH  anchor  or  grapple,  suspended  from  a  cord,  by  means  of  which  he 
can  seize  upon  some  terrestrial  object  when  he  comes  near  the  earth. 
When  the  anchor  is  made  fast,  the  aeronaut  draws  down  the  balloon 
by  pulling  upon  the  cord.  The  anchor,  the  sand  bags,  and  the 
wicker  car,  are  represented  on  the  ground  in  Fig.  105. 


How  does  the  aeronaut  make  fast  to  the  earth  in  descending  f 


POPULAK     PHYSICS.  153 

At  the  top  of  the  balloon  is  a  valve  kept  closed  by  a  spring;  it 
can  be  opened  by  means  of  a  string  descending  through  the  balloon 
to  the  car  of  the  aeronaut.  When  he  wishes  to  descend,  he  opens 
the  valve,  and  allows  a  portion  of  the  gas  to  escape.  To  ascertain 
whether  he  is  ascending  or  descending,  the  aeronaut  is  provided  with 
a  barometer ;  when  ascending,  the  barometric  column  falls,  and  when 
descending,  it  rises.  By  means  of  the  barometer  the  height  at  any 
time  may  be  determined. 


The  Parachute. 

143.  A  PARACHUTE  is  an  apparatus  by  means  of  which 
an  aeronaut  may  abandon  his  balloon,  and  descend  slowly 
to  the  earth. 

The  form  and  construction  of  a  parachute  is  shown  in 
Fig.  10G.  It  consists  of  circular  piece  of  cloth,  15  or  16  feet 
in  diameter,  presenting,  when  spread,  the  form  of  a  huge 
umbrella.  The  ribs  are  made  of  cords,  which,  being  con 
tinued,  are  attached  to  a  wicker  car,  as  shown  in  the  figure. 

When  the  aeronaut  wishes  to  descend  in  the  parachute,  he  enters 
the  car  and  detaches  the  parachute  from  the  balloon.  At  first  he 
descends  with  immense  rapidity,  but  the  air  soon  spreads  the  cloth, 
and  then  acting  by  its  resistance,  the  velocity  is  diminished,  and  the 
aeronaut  reaches  the  earth  without  injury.  A  hole  is  made  at  the 
centre  of  the  parachute,  which,  by  allowing  a  part  of  the  compressed 
air  to  escape,  directs  the  descent  and  prevents  violent  oscillations 
that  might  prove  dangerous. 

The  parachute  was  first  tried  by  BLANCHARD,  who  placed  a  dog  in 
the  car,  and  detached  it  from  the  balloon.  A  whirlwind  arrested 
its  descent  and  carried  it  up  above  the  clouds,  where  BLANCHARD 
soon  after  fell  in  with  it,  to  the  great  joy  of  the  poor  animal.  A 
current  again  separated  the  two  voyageurs,  but  both  reached  the 
earth  in  safety,  the  dog  being  the  last  to  descend. 

J.  GARNERIN  was  the  first  man  who  ventured  to  descend  in  a 
parachute,  which  he  did  by  detaching  himself  from  a  balloon  at  the 


What  is  the  vise  of  the  valve  at  the  top?     What  is  the  use  of  the  barometer f 
(143.)  What  is  a  Parachute?    Describe  it     Explain  it*  line  and  action. 


BALLOONING. 


BALLOONING.  155 

height  of  a  thousand  yards   above  the  surface    of  the  earth.     He 
descended  in  safety. 


Remarkable  Balloon  Ascensions. 

144.  The  first  ascension  was  made  in  October,  1783.  by  DK 
KOZIER.  His  balloon  was  filled  with  heated  air,  and  was  confined  by 
a  rope,  so  that  he  only  rose  to  a  height  of  about  a  hundred  feet.  In  the 
following  year  DE  ROZIER  and  D'ARLANDES  ascended  in  a  fire  balloon 
from  the  Bois  de  Boulogne,  and  after  a  voyage  of  twenty-five  minutes 
they  descended  on  the  other  side  of  Paris.  In  a  subsequent  ascent 
DE  ROZIER  lost  his  life  in  consequence  of  his  balloon  taking  fire.  In 
1785,  BLANCHARD  and  JEFFRIES  crossed  the  English  Channel  from 
Dover  to  Calais.  During  the  voyage  they  had  to  throw  overboard 
all  of  their  ballast,  then  their  instruments,  and  finally  their  clothing, 
to  lighten  the  balloon.  In  1804,  GAY  LUSSAC  oscended  to  the  height 
of  23,000  feet  above  the  level  of  the  sea.  At  this  height  the  baro 
metric  column  fell  to  12.6  inches,  and  the  thermometer,  which  at 
the  surface  of  the  earth  was  31°,  fell  to  9£°  below  0. 

At  such  heights,  substances  which  absorb  moisture,  like  paper  and 
parchment,  become  dry  and  crisp  as  if  h  \ated  in  an  oven,  respira 
tion  becomes  difficult,  and  the  circulation  is  quickened  on  account  of 
the  rarefaction  of  the  air.  GAY  LUSSAC  relates,  that  his  pulse  rose 
from  66  to  120.  The  sky  becomes  almost  black,  and  the  silence 
that  prevails  is  frightful.  After  a  voyage  of  six  hours,  GAY  LUSSAC 
descended,  having  travelled  about  ninety  miles. 

On  the  1st  of  July.  1859,  Messrs.  WISE,  LA  MOUNTAIN.  GAGER, 
and  HYDE,  ascended  from  St.  Louis,  Mo.,  and  descended  at  Hender 
son,  Jefferson  Co..  N.  Y.,  having  travelled  1150  miles  in  a  little  less 
than  twenty  hours,  or  about  fifty-seven  miles  per  hour.  This  is  the 
most  celebrated  voyage  on  record. 

During  the  recent  siege  of  Paris  balloons  were  successfully  em 
ployed  as  a  means  of  communication  between  the  forces  within  the 
city  and  those  without  the  lines  of  the  enemy.  Balloons  have  also  been 
used  for  making  observations  in  the  higher  regions  of  the  atmosphere. 

(144.)  Describe  some  of  the  most  remarkable,  Balloon  Ascensions.  Tliat  of 
ROZIER.  Of  BLANOHARD  and  JEFFRIES.  Of  GAY  LITSSAC.  What  effect  has  the 
atmosphere  at  great  elevations  f  Describe  the  great  American  voyage.  Uses. 


CHAPTER    IV. 

ACOUSTICS. 
1.  —  PRODUCTION      AND      PROPAGATION      OF      SOUND. 

Definition  of  Acoustics. 

145.  ACOUSTICS  is  that  branch  of  Physics  which  treats 
of  the  laws  of  generation  and  propagation  of  sound. 

Definition  of  Sound. 

146.  SOUND  is  a  motion  of  matter  capable  of  affecting 
the  ear  with  a  sensation  peculiar  to  that  organ. 

Sound  is  caused  by  the  vibration  of  some  body,  and  is 
transmitted  by  successive  vibrations  to  the  ear.  The  origi 
nal  vibrating  body  is  said  to  be  sonorous.  A  body  which 
transmits  sound  is  called  a  medium.  The  principal  medium 
of  sound  is  the  atmosphere ;  wood,  the  metals,  water,  <fcc., 
are  also  media. 


Let  us  take,  for  illustration,  a  stretched  cord  which  is  made  to 
vibrate  by  a  bow,  as  in  a  violin,  for  example.  When  the  cord  is 
drawn  from  its  position  of  rest.  «c6,  Fig.  107,  to  the  position  adb. 
every  point  of  the  cord  is  drawn  from  its  position  of  equilibrium : 

(145.)  What  is  Acoustics?  ( 146.)  What  is  Sound  ?  What  is  its  cause  ?  How  is 
it  transmitted  ?  What  is  a  sonorous  body?  A  medium?  Examples.  Explain  the 
vibrating  cord. 


PRODUCTION  AND  PKOPAGATION  OF  SOUND. 


157 


when  it  is  abandoned,  it  lends,  by  virtue  of  its  elasticity,  to  return  to 
its  primitive  state.  In  returning  to  this  position,  it  does  so  with  a 
velocity  that  carries  it  past  acb  to  aeb,  from  which  it  returns  again 
nearly  to  adb,  and  so  on  vibrating  backward  and  forward,  until,  after 
a  great  number  of  oscillations,  it  at  length  comes  to  rest. 

Sound-waves  in  Air.— Mode  of  Propagation. 
147.  Sound-waves  are  produced  in  the  air  by  the  vibra 
tion  of  some  sonorous  body.  When  the  body  moves  for 
ward  it  strikes  the  air  in  front  of  it  and  condenses  a  stratum 
whose  thickness  depends  on  the  rapidity  of  vibration ;  the 
particles  of  this  stratum  impart  the  condensation  to  those 


Fig.  108. 

of  the  next,  and  these  in  turn  to  those  of  the  next,  and  so 
on ;  the  condensation  thus  transmitted  outward  is  called 
the  condensed  pulse.  When  the  body  moves  backward,  the 
air  in  front  of  it  follows  and  produces  rarefaction  in  a 
stratum  whose  thickness  depends  on  the  rapidity  of  vibra 
tion ;  this  causes  a  backward  movement  and  consequent 
rarefaction  in  the  next  stratum,  which  is  transmitted  to 
the  next,  and  so  on ;  the  rarefaction  thus  propagated  out 
ward  is  called  the  rarefied  pulse.  Each  complete  vibration 
of  the  sonorous  body  generates  a  condensed  and  a  rarefied 
pulse,  and  these  taken  together  constitute  a  sound-wave. 


(1  47.)  Describe  the  mode  of  sound  propagation  in  tho  air. 


158  POPULAR    PHYSICS. 

If  the  vibrations  are  continuous,  a  series  of  sound-waves  are 
generated  travelling  outward  in  the  form  of  spherical  shells, 
as  shown  in  Figure  108. 

The  rate  at  which  the  sound-wave  travels  is  the  velocity  of  sound ; 
the  distance  through  which  it  travels  in  the  time  of  one  vibration  of 
the  sonorous  body  is  the  wave  length;  hence  the  wave  length  is 
always  equal  to  the  velocity  of  sound  divided  by  the  number  of 
vibrations  in  one  second.  The  form  of  the  sound-wave  is  transmitted 
through  the  air,  but  the  individual  particles  of  air  simply  oscillate  to 
and  fro  in  the  direction  of  wave  propagation,  moving  forward  on  the 
passage  of  the  condensed  and  backward  on  the  passage  of  the  rare- 
tied  pulse;  the  distance  through  which  each  particle  oscillates  is 
called  the  amplitude  of  vibration  of  the  particle. 

Any  two  particles  situated  on  a  line  in  the  direction  of  propagation, 
and  a  distance  from  each  other  equal  to  a  "wave  length,  are  always 
moving  in  the  same  direction  and  with  equal  velocities ;  such  parti 
cles  are  said  to  be  in  the  same  phase.  All  the  particles  of  any  wave 
that  are  in  the  same  phase  are  on  the  surface  of  a  sphere,  which  is 
called  a  wave  fro  fit. 

Superposition  of  Sound-waves. 

148.  It  is  to  be  remarked  that  many  sounds  may  be 
transmitted  through  the  air  simultaneously.  This  shows 
that  the  sound-waves  cross  each  other  without  modification. 
In  listening  to  a  concert  of  instruments,  a  practiced  ear  can 
detect  the  particular  sound  of  each  instrument. 

Sometimes  an  intense  sound  covers  up  or  dnrwns  a  more  feeble 
one ;  thus,  the  sound  of  a  drum  might  drown  that  of  the  human  voice. 
Sometimes  feeble  sounds,  which  are  too  faint  to  be  heard  separately, 
by  their  union  produce  a  sort  of  murmur.  Such  is  the  cause  of  the 
murmur  of  weaves,  the  rustling  sound  of  a  breeze  playing  through 
the  leaves  of  a  forest,  and  the  indistinct  hum  of  a  distant  city. 

It  has  been  shown  that  two  sound-waves  may,  under  certain  cir 
cumstances,  neutralize  each  other,  producing  silence. 

What  is  the  velocity  of  sound?  The  wave  length?  The  amplitude  of  vibration 
of  a  particle?  Its  direc'ion  ?  A  wave  front  f  (148.)  Do  sound-waves  interfere 
with  each  other V  progress  ?  How  shown  ?  Explain  the  nwnrur  of  leaves.  Waves. 


PRODUCTION    AND    PROPAGATION     OF     SOUND.  159 

Sound  is  not  propagated  in  a  Vacuum. 

149.  That   some   medium   is   necessary   for  the  trans 
mission  of  sound,  may  be  shown  by  the  following  experi 
ment. 

In  a  glass  globe  with  a  stop-cock,  is  suspended  a  bell,  as 
shown  in  Fig.  109.  When  the  globe  is 
shaken,  the  sound  of  the  bell  is  distinctly 
heard.  If  the  air  be  exhausted  from  the 
globe,  no  sound  is  heard  when  the  globe 
is  shaken. 

This   experiment   may  be   performed 
otherwise  as  follows  : 

A  bell  is  placed  under  the  receiver  of 
an   air-pump,  provided  with    a    striking 
apparatus  set  in  motion  by  clock-work. 
Before  the  air  is  exhausted,  the  strokes 
of  the  hammer  on  the  bell  are  distinctly  heard,  but  as  the 
air  is  exhausted  the  sound  becomes  fainter  and  fainter,  till 
at  last  it  ceases  to  be  heard. 

For  the  complete  success  of  this  experiment,  the  bell  and 
clock-work  should  be  placed  upon  a  cushion,  of  some  sub 
stance  which  does  riot  readily  transmit  sound. 

In  ascending  high  mountains,  the  air  becomes  rarefied,  and  a  cor 
responding  diminution  in  the  intensity  of  sounds  is  observed. 
SAUSSURE.  on  firing  a  pistol  on  the  summit  of  Mt.  Blanc,  reports 
that  it  produced  only  a  feeble  sound,  like  that  heard  on  breaking  a 
stick. 

Propagation   of  Sound  in  Liquids   and   Solids. 

150.  Sound  is  transmitted,  not  only  by  gases,  but  also  by 
liquids  and  solids.     Divers  hear  sounds  from  the  shore  when 


(149.)  How  is  it  shown  that  sound  is  not  transmitted  in  a  vacuum  ?  Another 
method  of  showing  the  same  thins:.  Effect  of  elevation  on  sound.  (  1 5O.)  How  is 
it  shown  that  liquids  and  solids  transmit  sounds? 


160  POPULAR     PHYSICS. 

under  water,  and  sounds  made  under  water  are  heard  on 
shore.  A  slight  sound  made  at  one  end  of  a  long  stick  of 
timber  is  distinctly  heard  by  an  ear  at  the  other  end,  even 
when  it  might  be  inaudible  at  an  equal  distance  through  the 
air. 

The  earth  transmits  sounds,  and  by  placing  the  ear  in  con 
tact  with  it,  sounds  may  be  distinguished  at  a  great  distance. 
This  method  of  hearing  approaching  footsteps  of  men  or 
animals,  is  well  understood  by  hunters.  In  the  construction 
of  subterranean  galleries  for  mining  purposes,  the  miner  is 
often  guided,  as  to  the  direction  he  should  take,  by  sounds 
transmitted  through  large  masses  of  earth  and  rock. 


Velocity  of  Sound  in  the  Air. 

That  sound  occupies  an  appreciable  time  in  passing 
from  point  to  point  may  be  shown  by  many  familiar  ex 
amples.  If  we  notice  a  man  cutting  wood  at  a  distance,  we 
perceive  that  his  axe  falls  some  time  before  the  sound  of  the 
blow  reaches  the  ear.  If  a  gun  is  discharged,  we  see  the 
flash  before  we  hear  the  report.  In  like  manner  the  flash 
of  lightning  is  seen  before  we  hear  the  thunder. 

In  1822,  a  number  of  scientific  men  undertook  a  series  of 
very  nice  experiments  to  determine  the  velocity  of  sound. 
They  placed  a  cannon  on  the  hill  of  Montlery,  near  Paris, 
and  another  on  a  plain  near  Ville-Juif,  the  distance  between 
them  being  61,047  feet.  At  each  station  twelve  discharges 
were  made  at  intervals  of  ten  minutes ;  the  discharges 
alternating  between  the  stations  at  intervals  of  five  minutes. 
Observers  placed  at  each  station  observed  the  intervals  of 
time  that  elapsed  between  seeing  the  flash  and  hearing  the 
report  of  the  cannon  at  the  other  station.  The  average 


Howls  It  shown  that  the  earth  transmits  sound?  Illustrate.  (151.)  How  is  it 
shown  that  sound  requires  an  appreciable  time  to  pass  from  place  to  place  ? 
Illustrate.  Explain  the  experiments  made  near  Paiis. 


PRODUCTION    AND    PROPAGATION    OF    SOUND.  161 

interval  was  5.4.6  seconds,  and  the  temperature  was  61°  F. ; 
the  actual  velocity  was  found  to  be  1118  ieet  per  second, 
which,  after  correcting  for  temperature,  gave  1090  ieet  per 
second  for  the  temperature  32°  F.  The  velocity  increases 
about  1  foot  per  second  for  each  degree  of  Fahrenheit. 

Velocity  of  Sound  in  Liquids  and  Solids. 

152.  Liquids  and  solids   transmit  sound  more  rapidly 
than  air.     Experiments  made  by  transmitting  sound  across 
the  Lake  of  Geneva,  in  Switzerland,  show  that  the  velocity 
of  sound  in  water  is  about  4700  feet  per  second,  which  is 
more  than  four  times  its  velocity  in  air. 

That  sound  travels  faster  in  iron  than  in  air,  may  be 
shown  by  placing  the  ear  at  one  extremity  of  a  long  iron 
bar  or  tube,  whilst  it  is  struck  on 'the  other  end  with  a 
hammer.  Two  sounds  will  be  heard,  the  first  transmitted 
through  the  iron,  and  the  second  through  the  air. 

Reflection  and  Refraction  of  Sound. 

153.  Sound-waves  in  air  emanating  from  a  point  move 
in  concentric  spheres ;  if  they  meet  an  obstacle,  they  are 
turned  back,  forming  a  new  set  of  waves  whose  common 
centre  is  as  far  behind  the  obstacle  as  the  generating  point 
is  in  front  of  it.     This  phenomenon  is  called  reflection,  and 
the  two  sets  of  waves  are  called  incident  and  reflected  waves. 
A  ray  of  sound  is  a  line  along  which  sound  acts;  it  is  nor 
mal,  or  perpendicular  to  the  wave  fronts,  and  consequently 
its  direction  passes  through  their  centre.     A  line  drawn  to 
any  point  of  the  reflecting  surface  from  the  centre  of  inci 
dent  waves,  is  an  incident  ray ;  a  line  drawn  through  the 
same  point  from  the  centre  of  reflected  waves,  is  a  reflected 
ray  ;  the  point  itself  is  the  point  of  incidence. 

What  is  the  velocity  of  sound  at  32°  P.  ?  At  what  rate  does  it  increase  with  the 
temperature  ?  (152.)  How  was  it  shown  that  sound  travels  faster  in  water  than 
in  air?  In  iron  than  air?  (153.)  What  is  reflection?  Incident  and  reflected 
waves  ?  Incident  and  reflected  rays  ? 


162 


POPULAK    PHYSICS. 


Let ./,  /,  be  incident  waves  whose  centre  is  C;  F,  W,  reflected 
waves  whose  centre  is  C';  and  PJV,  a  normal  to  the  reflecting  sur 
face,  AB,  at  the  point  P. 
Then  is  CP  an  incident 
ray;  PR  a  reflected  ray; 
and  P  is  the  point  of  inci 
dence.  The  angle,  CPN,  is 
called  the  angle  of  incidence, 
and  NPR  is  called  the  angle 
of  reflection.  The  incident 
and  reflected  rays  lie  in  a 
plane  normal  to  the  reflect 
ing  surface  at  the  point  of 
incidence,  and  the  angles  of  incidence  and  reflection  are  equal. 

Refraction  is  the  change  of  direction  experienced  by  a 
ray  when  it  passes  obliquely  from  one  medium  to  another. 

Let  /,  /,  be  waves  whose  centre  is  (7,  incident  on  the  surface  AB, 
which   separates  two  media,  and  let  TV,  TF,  be  modified  waves  in 

the  second  medium.  If 
sound  travel  at  different 
rates  in  the  two  media,  the 
modified  waves  in  the  sec 
ond  medium  will  have  their 
centre  at  some  point,  C",  on 
the  perpendicular  to  AE 
through  C.  If  the  velocity 
in  the  second  medium  be 
less  than  in  the  first,  C'  will 
be  further  from  AB  than  (7, 
otherwise  the  point  C'  will 
be  between  C  and  AB.  Sup 
pose  the  former  case,  and  let  NN'  be  a  normal  to  the  deviating  sur 
face  AB  at  P;  then  is  CP  an  incident  ray ;  PR  a  refracted  ray; 
CPN  the  angle  of  incidence  ;  N'PR  the  angle  of  refraction  ;  and  GPC' 
is  the  amount  of  refraction.  The  angles  of  incidence  and  refraction 
are  in  a  plane  normal  to  the  deviating  surface  at  the  point  of  incidence, 
and  the  sine  of  the  angle  of  incidence  is  equal  to  the  sine  of  the  angle  of 
refraction  multiplied  by  a  constant  quantity,  called  the  refractive  index. 


Fig.  HOa. 


What  relation  exists  between  the  incident  and  reflected  rays  ?    Define  refraction. 
What  is  the  incident  ray  ?    The  refracted  ray  ?    The  relation  between  them  ? 


PRODUCTION    AND    PROPAGATION    OF    SOUND.  163 


Echoes. 

154.  An  echo  is  a  sound  repeated  by  reflection.    In  order 
that  the  echo  of  any  sound  may  be  clearly  heard,  the  reflect 
ing  surface  ought  to  be  so  far  distant  from  the  listener  as 
to  require  at  least  the  fifth  of  a  second  for  sound  to  travel 
to  it  and  return. 

It  is  not  possible  to  pronounce  or  to  hear  distinctly  more  than  five 
syllables  in  a  second.  The  velocity  of  sound  being  1090  feet  per 
second,  it  follows  that  sound  travels  218  feet  in  one  fifth  of  a  second. 
If.  then,  an  obstacle  be  placed  at  the  distance  of  109  feet,  sound  will 
go  to  it  and  return  in  one  fifth  of  a  second.  At  the  distance  of  109 
feet,  the  last  syllable  only  of  the  echo  will  reach  the  ear  after  the 
sentence  is  pronounced.  Such  an  echo  is  called  monosyllabic.  If 
the  echo  takes  place  from  an  obstacle  at  a  distance  of  2 1 8  feet,  we 
hear  two  syllables ;  that  is,  the  echo  is  dissyllabic.  At  distances  of 
327  feet,  the  echo  is  trisyllabic,  and  so  on. 

Sound  may  be  reflected  from  several  objects  situated  in 
different  directions  and  at  different  distances.  Such  echoes 
are  called  multiple  echoes.  It  is  said  that  at  a  place  three 
leagues  from  Verdun,  a  multiple  echo  formed  by  parallel 
walls  fifty  or  sixty  yards  apart,  repeats  a  sound  twelve  times. 
At  the  chateau  of  Simonnetta,  in  Italy,  there  is  an  echo 
which  repeats  the  report  of  a  pistol  from  forty  to  fifty 
times. 

Echoes  modify  the  tones  of  sound.  Some  repeat  sounds 
with  a  roughened,  others  with  a  softened  tone ;  some  with 
a  sneering,  others  with  a  plaintive  accent. 

Resonance. 

155.  When  sounds  are  reflected  from  obstacles  at  a  less 
distance  than  109  feet,  the  reflected  sound  is  superposed 

What  is  an  echo  ?  Its  cause  ?  Explain  the  monosyllabic,  dissyllabic,  and  trisyl 
labic  echoes.  What  arc  multiple  echoes  ?  Examples.  What  effect  have  echoes  on 
the  tone  of  a  sound  ?  (155.)  What  is  a  Resonance  ? 


164:  POPULAR    PHYSICS. 

upon  the  direct  one,  giving  rise  to  a  strengthened  sound, 
which  is  called  a  Resonance. 

It  is  the  resonance  from  the  walls  of  a  room  that  makes  it  easier 
to  speak  in  a  closed  apartment  than  in  the  open  air.  The  resonance 
is  more  clearly  perceived  when  the  walls  are  elastic.  In  rooms 
where  there  are  carpets,  curtains,  stuffed  furniture,  and  the  like,  the 
sound-waves  are  broken  up.  and  the  resonance  is  diminished  ;  but  in 
houses  where  there  is  no  furniture,  the  resonance  is  strengthened. 
Hence  it  is,  that  the  sound  of  voices,  footsteps,  and  the  like,  is  so 
strongly  marked  in  deserted  and  unfurnished  buildings. 

Intensity  of  Sound. 

156.  The  intensity  of  sound  depends  on  the  force  with 
which  it  strikes  the  ear.    It  varies  very  nearly  as  the  square 
of  the  amplitude  of  vibration  of  the  serial  particles.     Some 
of  the  causes  that  modify  the  intensity  of  sound  are  noticed 
in  the  following  article. 

Causes  that  modify  the  Character  of  Sound, 

157.  The  following  are  some  of  the  causes  that  modify 
the  intensity  and  rate  of  propagation  of  sound  : 

1.  It  is  shown  by  theory  and  confirmed  by  experiment, 
that  the  intensity  of  sound  diminishes   as  the  square  of  the 
distance  from  the  sonorous  body  increases. 

This  is  expressed  by  saying  that,  the  intensity  of  sound 
varies  inversely  as  the.  square  of  the  distance  from  the 
sonorous  body. 

2.  The  intensity  of  sound  diminishes  with  the  amplitude 
of  the  vibration  of  the  aerial  particles. 

When  a  cord  vibrates,  the  sound  is  observed  to  diminish  as  the 
vibrations  become  smaller,  and  when  the  vibrations  cease,  the  sound 


Illustrate  by  examples.    ( 156.)  What  is  Intensity?    On  what  depend  ?    (157.) 
What  are  the  laws  of  intensity  ?    1.  Effect  of  distance  ?    2.  Amplitude  of  vibration? 


PRODUCTION    AND    PROPAGATION    OF    SOUND.  165 

is  no  longer  heard.  The  amplitude  of  vibration  of  the  sonorous 
body  determines  the  length,  or  amplitude  of  the  vibrations  of  the  aerial 
particles. 

3.  The  density  of  the  air  modifies  sound.      When   the 
air  is  rarefied,  the  intensity  is  diminished.     This  fact  has 
been  shown  by  the  experiment  of  a  bell  in  an  exhausted 
receiver. 

The  presence  of  watery  vapor  in  the  air  also  modifies  sound,  that 
substance  being  a  good  conductor  of  sound.  When  the  air  is  cooled, 
it  becomes  more  dense,  hence,  sounds  are  louder  in  cold  than  in 
warm  weather. 

4.  The  wind  modifies  sound.     The  velocity  of  sound  is 
increased  or  diminished  by  the  velocity  of  the  wind,  accord 
ing  as  the  direction  of  the  wind  conspires  with  or  opposes 
the  propagation. 

The  effect  of  the  wind  is  to  move  the  whole  mass  of  air,  carrying 
along  the  sound-waves  unaltered. 

5.  Sound  is  increased  in  intensity  when  the  sonorous  body 
is  in  contact  with,  or  even  in  the  neighborhood  of  another 
body  capable  of  vibrating  in  unison  with  it. 

Hence,  the  sound  of  a  vibrating  cord  is  reinforced  or  strengthened 
by  stretching  it  over  a  thin  box  filled  with  air,  as  in  the  violin.  In 
this  case  the  air  in  the  body  of  the  violin  vibrates  in  unison  with 
the  cord.  The  ancients  placed  in  their  theatres  vessels  of  brass,  to 
reinforce  and  strengthen  the  voices  of  the  actors. 

Intensity  of  Sounds  in  Tubes. 

158.  When  a  sound  is  transmitted  through  a  tube,  the 
sound-waves  can  not  diverge  laterally,  and  consequently  the 


3.  Density  of  the  air  ?  Illustrate.  4.  How  does  wind  modify  sound  ? 
5.  Effect  of  a  neighboring  sonorous  body  ?  Illustrate,.  (  158.)  What  effect  has  a 
tube  on  sound? 


166 


POPULAR     PHYSICS. 


sound  is  transmitted  to  a  great  distance  without  much  loss 
of  intensity. 

M.  BIOT  was  able  to  carry  on  a  conversation  in  a  low  tone  through 
a  tube  a  thousand  feet  in  length.  He  says  that  the  sound  was 
transmitted  so  well,  that  there  was  but  one  way  to  avoid  being 
heard,  and  that  was  not  to  speak  at  all. 


Fig.  111. 

This  property  of  tubes  is  utilized  in  hotels  and  dwelling-houses, 
for  transmitting  messages  from  one  story  to  another.  The  tubes 
employed  for  this  purpose  are  called  speaking  tubes.  The  method 
of  employing  the  speaking  tube,  is  illustrated  in  Fig.  111. 


The  Speaking  Trumpet. 

159.     The  SPEAKING  TRUMPET,  as  its  name  implies,  is 
a  conical  tube   employed  to  transmit  the  voice  to  a  great 


BIOTS  experiment.    Practical  applications.    (159.)  What  is  a  Speaking  Trum 
pet  ? 


PRODUCTION    AND    PROPAGATION     OF     SOUND. 


167 


distance.     It  is  used  by  firemen  and  by  mariners,  as  shown 
in  Fig.  112. 


Fig.  112. 

By  means  of  the  speaking  trumpet,  the  voice  of  the  captain  can 
be  heard  above  the  noi.se  of  the  winds  and  waves  in  a  tempest. 
According  to  Father  KIRCHER.  ALEXANDER  THE  GREAT  employed  a 
speaking  trumpet  in  commanding  his  armies. 

The  effect  of  the  speaking  trumpet  has  been  explained  by  succes 
sive  reflections  of  sound-waves  from  the  sonorous  material  of  which 
the  instrument  is  composed,  by  virtue  of  which  the  voice  istrans- 
mitted  only  in  the  direction  of  the  tube. 

But  the  fact  is,  that  sound  is  transmitted  in  all  directions,  which 
would  indicate  that  its  effect  should  be  attributed  to  a  reinforcement 
of  the  voice  fey  the  vibration  of  the  column  of  air  contained  in  the 
trumpet,  according  to  the  principle  that  sound  is  reinforced  by  an 
auxiliary  vibrating  body. 


is  the,  effect  ofthe,  speaking  trumpet  explained  f 


108  POPULAR    PHYSICS. 

The  Ear  Trumpet. 

16O.     The   EAR   TRUMPET   is   a   trumpet   employed  by 
persons  whose  hearing  is  defective,  as  shown  in  Fig.  11-3. 


Fig.  113. 


It  is  simply  the  speaking  trumpet  reversed.  It  serves  to 
collect  and  concentrate  the  sound-waves,  which  are  thus 
enabled  to  produce  a  more  powerful  impression  on  the  drum 
of  the  ear.  The  shape  of  the  ear  in  man  and  in  animals  is 
such  as  to  perform  the  function  of  the  trumpet. 


II.  —  MUSICAL       SOUNDS. 

Difference  between   a   Musical   Sound  and  a  Noise. 

161.  A  MUSICAL  SOUXD  results  from  a  succession  of 
vibrations  of  equal  duration.  Such  vibrations  are  called 
isochronal. 


(  1  GO.)  What  is  an  Ear  Trumpet?    How  does  it  differ  from  the  speaking  trumpet? 
A/hat  is  its  use  ?    (  161.)  What  is  a  Musical  Sound  ? 


MUSICAL    SOUNDS.  169 

results  from  a  single  impulse,  or  from  a  succession 
of  vibrations  of  unequal  duration.  Thus,  the  crack  of  a 
whip,  the  discharge  of  a  pistol,  the  rattling  of  thunder,  or 
the  roar  of  the  waves  of  the  ocean,  are  destitute  of  musical 
value,  and  are  simply  noises. 

Fitch  of  Sounds.— Music. 

162.  The  PITCH  of  a  musical  sound  depends  upon  the 
frequency  of  the  vibrations.     Those  sounds  which  result  from 
very  rapid  vibrations,  are  called  acute,  whilst  those  which 
arise  from  very  slow  vibrations,  are  called  grave. 

The  terms  acute  and  grave  are  relative  •  thus,  a  given  sound  may 
be  acute  with  respect  to  a  second,  whilst  it  is  grave  with  respect  to 
a  third  ;  thus,  a  sound  which  corresponds  to  160  vibrations,  is  acute 
with  respect  to  one  corresponding  to  80  vibrations,  and  grave  with 
respect  to  one  corresponding  to  320  vibrations  per  second.  A  well 
arranged  and  happy  combination  of  grave  and  acute  sounds  accord 
ing  to  the  principles  of  harmony,  constitutes  music. 

Limits  of  perceptible   Sounds. 

163.  M.  SAVAKT  investigated  the  subject  of  sound  with 
respect  to  the  number  of  vibrations,  corresponding  to  the 
most   grave  and   acute   sounds  perceptible  by  the  human 
ear,  by  means  of  an  apparatus  devised  for  that  purpose. 

As  the  result  of  his  investigations,  he  concluded  that  the 
gravest  perceptible  sound  was  produced  by  16  vibrations 
per  second,  and  the  most  acute  by  48,000  vibrations  per 
second.  Allowing  1090  feet  as  the  velocity  of  sound,  we 
find  for  the  length  of  the  waves,  corresponding  to  the 
gravest  rounds,  68  feet,  and  for  the  length  corresponding 
to  the  most  acute  sounds,  a  little  more  than  a  quarter  of 
an  inch. 

What  is  a  Noise  ?  1 62.  What  does  Pitch  depend  upon  ?  What  is  an  acute  sound  ? 
A  grave  one?  Illustrate  ly  examples.  What  is  muaict  (163.)  Who  investi 
gated  the  limits  of  audible  sounds  ?  Give  the  results  of  his  investigation. 

ft 


170  POPULAR     PHYSICS. 

The  limits  of  sounds  employed  in  music  are  much  narrower, 
especially  in  singing.  SAVART  gives  for  the  gravest  sounds  of  the 
male  voice,  190  vibrations  per  second,  and  for  the  female  voice,  572. 
For  the  most  acute  sounds  of  the  male  voice  he  gives  678  vibrations 
per  second,  and  for  the  female  voice,  1606. 

Two  sounds,  corresponding  to  the  same  number  of  vibrations  pe 
second,  are  in  unison. 


Musical  Scale.— Gamut. 

164.  The  ear  not  only  distinguishes  between  given 
sounds — which  is  most  grave,  and  which  is  most  acute — but 
it  also  appreciates  the  relations  between  the  number  of  vibra 
tions  corresponding  to  each.  We  can  not  recognize  whether 
for  one  sound  the  number  of  vibrations  is  precisely  two, 
three,  or  four  times  as  great  as  for  another,  but  when  the 
number  of  vibrations  corresponding  to  two  successive  or 
simultaneous  sounds  have  to  each  other  a  simple  ratio,  these 
sounds  excite  an  agreeable  impression,  which  varies  with 
the  relation  between  the  two  sounds. 

From  this  principle  there  results  a  series  of  sounds  char 
acterized  by  relations  which  have  their  origin  in  the  nature 
of  our  mental  organization,  and  which  constitute  what  is 
called  a  Musical  Scale. 

In  this  scale  sounds  recur  in  the  same  order  in  groups  of  seven. 
Each  group  constitutes  what  is  called  a  gamut  of  seven  notes.  The 
notes  are  named,  do,  re,  mi,  fa,  sol,  la,  si,  but  they  are  usually  denoted 
by  the  letters,  C,  D,  E,  F,  G,  A,  B.  The  relation  between  these  notes 
is  given  in  the  table  below,  n  denoting  the  number  of  vibrations 
corresponding  to  the  note  C : 


c, 
», 

91 

E,            F, 
5              4 

471'          3W) 

G,           A, 
3             5 

271'          3n' 

B, 

15 

C; 

2n, 

What  are  the  limits  in  singing  f     When  are  sounds  in  'unison  ?    (1 64.)  What  is 
a  Musical  Scale  ?     Whqt  is  a  gamut?    How  are  the  notes  named  ? 


MUSICAL   SOUNDS.  171 

The  number  of  vibrations  corresponding  to  any  note  may  be  meas 
ured  by  an  instrument  called  a  siren. 

Intervals. — Accords. 

165.  An  interval  is  the  ratio  of  the  number  of  vibrations 
corresponding  to  any  note  to  the  number  corresponding  to 
some  higher  note. 

The  intervals  between  consecutive  notes,  called  seconds,  is  given  in 
the  following  table : 

C  to  D,    D  to  E,    E  to  F,    F  to  G,    G  to  A,    A  to  B,    B  to  C ; 

9  10  16  9  10  9  16 

8'  IP  15'  8'  9'  8'  15' 

If  the  interval  comprise  two,  three,  four,  &c.,  seven  notes,  it  is  called, 

a  third,  a  fourth,  a  fifth,  &c.,  an  eighth,  or  an  octave;  thus,  the  interval 

between  C  and  E  is  a  third,  and  is  equal  to  £ ;  the  interval  from  C  to 

F  is  a  fourth,  and  is  equal  to  $ ;  the  interval  from  any  note  to  the 

next  note  of  the  same  name  is  an  octave,  and  is  always  equal  to  2. 

The  coexistence  of  several  sounds  is  called  an  accord. 
When  the  ear  can  distinguish,  without  fatigue,  the  relation 
between  two  sounds,  which  is  the  case  when  this  relation  is 
simple,  the  coexistence  of  these  sounds  is  called  a  conso 
nance  ;  when  the  ear  is  painfully  affected  by  the  coexist 
ence,  it  is  called  a  dissonance. 

The  most  simple  accord  is  the  unison,  in  which  the  number  of 
vibrations  are  equal :  then  cornes  the  octave,  in  which  the  number  of 
vibrations,  corresponding  to  one  sound,  is  double  that  corresponding 
to  the  other ;  then  the  fifth,  in  which  the  numbers  are  as  3  to  2  ; 
then  the  fourth,  in  which  the  numbers  are  as  4  to  3  ;  and  finally 
the  third,  in  which  the  ratio  is  that  of  5  to  4. 

When  the  numbers  of  vibrations  corresponding  to  three  simultane 
ous  sounds,  are  as  4,  5,  and  6,  the  combination  is  called  a  perfect 
accord.  For  example,  the  notes  c,  E,  G,  form  a  perfect  accord,  as 


(165.)  What  is  an  Interval?  What  is  a  third,  fourth,  fifth,  sixth,  seventh, 
octave?  What  is  an  Accord ?  A  Consonance?  A  Dissonance?  What  is  the  sim 
plest  Accord?  The  next  simplest?  Next  three  in  order t  What  is  a  perfect 
accord  ?  Example. 


172 


POPULAK    PHYSICS. 


do  the  notes    G,    B,    D,     These  accords  produce  upon  the  ear  the 
most  agreeable  sensation. 

The  Tuning  Fork. 

166.  A  TUNING  FOKK  is  an  instrument  used  in  tuning 
musical  instruments  of  fixed  sounds,  like  the  piano. 

It  consists  of  a  plate  of  steel,  bent  into  the  shape  of  the 
letter  U,  mounted  upon  a  wooden  box,  as  shown  in  Fig.  114. 
The  wooden  box  is  open  at  one  extremity,  and  serves  to 


Fig.  114. 

reinforce  the  sound,  which  would  otherwise  be  feeble.  The 
fork  is  made  to  sound  by  drawing  across  one  of  its  branches 
a  violin  bow,  or  by  straining  the  branches  apart  by  a  wedge 
of  wood  or  metal,  and  then  suddenly  withdrawing  it,  or 
finally,  by  striking  one  of  the  branches  with  a  ?olid  body. 
The  tuning  fork  is  usually  constructed  so  as  to  sound  the  A, 
which  corresponds  to  856  vibrations  per  second. 


(  1 66.)  What  is  a  Tuning  Fork  ?    Describe  it.    How  is  it  marie  to  sound  ? 


MUSICAL    SOUNDS.  173 

Transverse    Vibrations  of  Cords. 

167.  We   have  already  seen  (Art.  146),  that  when  a 
stretched  cord  is  drawn  from  its  position  of  equilibrium 
and  abandoned,  it  returns  to  its  position  of  rest  by  a  suc 
cession- of  continually  decreasing  vibrations. 

Cords  used  in  musical  instruments  are  generally  made  of 
catgut,  or  of  twisted  wires.  They  are  made  to  vibrate  by 
drawing  a  bow  across  them,  as  in  the  violin ;  by  drawing 
them  aside,  as  in  the  harp ;  or  by  percussion  writh  little 
hammers,  as  in  the  piano.  In  all  of  these  cases,  the  vibra 
tions  are  transversal,  that  is,  the  movements  take  place 
perpendicularly  to  the  direction  of  the  cord. 

Laws  of  Transversal   Vibrations   of  Cords. 

168.  The  number  of  vibrations  of  a  stretched  cord  in 
nny  given  time,  as  in  one  second,  for  example,  depends  upon 
its  length,  its  thickness,  its   tension,  and  its  density.     The 
following  are  the  laws  that  govern  the  number  of  vibrations 
in  a  fixed  time  : 

1.  The  tension  being  constant   the  number  of  vibrations 
varies  inversely  as  its  length. 

If  a  given  cord  makes  18  vibrations  per  second,  it  will  make  36  if 
its  length  be  reduced  to  one  half,  54  if  its  length  be  reduced  to  one 
third,  and  so  on.  This  property  is  utilized  in  the  violin.  By  apply 
ing  the  finger,  we  virtually  reduce  the  length  of  the  vibrating  portion 
at  pleasure. 

2.  The  tension  and  length  being  the  same,  the  number  of 
vibrations  varies  inversely  as  its  diameter. 

Small  cords  vibrate  more  rapidly  than  large  ones,  and  con 
sequently  render  more  acute  sounds.  A  cord  of  any  given  size 


( 1 67.)  Of  what  are  musical  cords  made  ?  How  set  in  vibration  in  different  instru 
ments  ?  (168.)  Upon  what  does  the  number  of  vibrations  of  a  cord  depend  ?  What 
is  the  first  law  ?  Illustrate.  The  second  law  ?  Illustrate. 


1 74:  POPULAK     PHYSICS. 

makes  twice  as  many  vibrations  as  one  of  double  the  size.     Other 
things  being  equal,  the  notes  rendered  differ  by  an  octave. 

3.  The  length  and  size  being  the  same,  the  number  of 
vibrations  varies  as  the  square  root  of  the  tension. 

If  a  cord  renders  a  given  note,  it  will,  if  its  tension  be  quadrupled, 
render  a  note  an  octave  higher,  and  so  on.  This  property  is 'utilized 
in  stringed  instruments  by  means  of  an  apparatus  for  increasing  or 
diminishing  the  tension  at  pleasure. 

4.  Other  things  being  equal,  the  number  of  vibrations 
varies  inversely  as  the  square  root  of  the  density. 

Dense  cords  render  graver  notes  than  those  of  less  density.  Small, 
light,  and  short  cords,  strongly  stretched,  yield  acute  notes.  Large, 
dense,  and  long  cords,  not  strongly  stretched,  yield  grave  notes. 

Verification  of  the   Laws  of  Vibration. 

169.  The  laws  enunciated  in  the  preceding  article  may 
be  verified  by  means  of  an  instrument  called  a  Sonometer, 
shown  in  Fig.  115. 


Fig.  115. 


The  sonometer  is  said  to  have  been  invented  by  PYTHAG 
ORAS,  about  600  years  before  our  era.     In  its  present  form, 

The  third  law  ?    Illustrate.    The  fourth  law  ?    Illustrate.    ( 1 69.)  How  may  the 
preceding  laws  be  verified  ?    What  is  a  Sonometer  ? 


MUSICAL     SOUNDS.  175 

it  consists  of  a  wooden  box  about  four  feet  in  length,  upon 
which  are  mounted  two  fixed  bridges,  A  and  B,  and  a 
movable  one,  D.  On  these  bridges,  two  cords,  CD  and 
AB,  fastened  firmly  at  one  end  and  passing  over  pulleys  at 
the  other  end,  are  stretched  by  means  of  weights,  P. 

Let  the  cords  be  exactly  alike  and  stretched  by  equal 
weights.  If  the  bridge  D,  be  moved  so  as  to  render  CD 
equal  to  one  half  of  AB,  the  notes  of  the  two  cords  will 
differ  by  an  octave  ;  that  is,  CD  will  vibrate  twice  as  fast 
as  AB.  If  CD  be  made  equal  to  one  third  of  AB,  by 
moving  the  bridge  D,  the  former  will  vibrate  three  times 
as  fast  as  the  latter,  and  so  on.  This  verifies  the  first  law. 
To  verify  the  second  law,  we  remove  the  bridge  D,  and 
use  -two  cords,  one  of  which  is  twice  as  large  as  the  other. 
It  will  be  found  that  the  notes  yielded  will  differ  by  an 
octave.  If  one  cord  be  taken  three  times  as  large  as  the 
other,  the  latter  will  be  found  to  vibrate  three  times  as 
fast  as  the  former. 

To  verify  the  third  law,  let  the  two  cords  be  alike,  and 
stretch  one  by  a  weight  four  times  as  great  as  that  employed 
to  stretch  the  other.  The  notes  will  differ  by  an  octave. 
If  the  stretching  force  in  one,  is  nine  times  that  in  the  other 
case,  the  former  will  vibrate  three  times  as  fast  as  the  latter, 
and  so  on. 

To  verify  the  fourth  law,  we  make  use  of  cords  equal  in 
length,  size,  and  equally  stretched,  but  of  different  densities. 
It  will  be  found  that  the  law  is  verified  in  each  case 

Stringed  Instruments. 

1TO.  All  stringed  instruments  of  music  are  constructed 
in  accordance  with  the  preceding  laws.  They  are  divided 
into  instruments  with  fixed  sounds,  and  instruments  with 
variable  sounds. 


Describe  it.     How  is  the  first  law  verified?     The  second?     The  third?     The 
fourth  ?    (1 7O.)  How  are  stringed  instruments  classed  ? 


176  POPULAR     PHYSICS. 

To  the  former  class  belong  the  piano,  the  harp,  &c.  They 
have  a  cord  for  each  note,  or  else  an  arrangement  is  made 
so  that  by  placing  the  finger  at  certain  points,  as  in  the 
guitar,  the  same  cord  may  be  made  to  render  several  notes 
in  succession. 

To  the  latter  class  belong  the  violin,  the  violoncello,  &c. 
They  are  provided  with  cords  of  catgut,  or  sometimes  of 
metal,  put  in  vibration  by  a  bow.  Various  arrangements 
are  made  for  regulating  the  notes,  such  as  increasing  the 
tension,  placing  the  finger  upon  the  cords,  and  the  like. 
These  instruments  are  difficult  to  play  upon,  and  require 
great  nicety  of  ear,  but  in  the  hands  of  skillful  players  they 
possess  great  power.  They  are  the  soul  of  the  orchestra, 
and  it  is  for  them  that  the  finest  pieces  of  music  have  been 
composed. 

Sound   from    Pipes. 

171.  When  the  air  in  &pipc,  or  hollow  tube,  is  put  into 
vibration,  it  yields  a  sound.     In  this  case,  it  is  the  air  Avhich 
is  the  sonorous  body,  the  nature  of  the  sound   depending 
upon  the  form  of  the  pipe  and  the  manner  in  which  the 
vibrations  of  its  contained  air  are  produced. 

To  produce  a  sound  from  a  pipe,  the  contained  air  must 
be  thrown  into  a  succession  of  rapid  condensations  and 
rarefactions,  which  is  effected  by  introducing  a  current  of 
air  through  a  suitable  mouth-piece.  Two  principal  forms  are 
given  to  the  mouth-piece,  in  one  of  which  the  parts  remain 
fixed,  and  in  the  other  there  is  a  movable  tongue,  '  called 
a  reed. 

Pipes  with  fixed  Mouth-pieces. 

172.  Pipes  writh   fixed   mouth-pieces   are   of  wood  or 
metal,  rectangular  or  cylindrical,  and  always  of  considerable 

Examples  of  each  class.  Which  are  most  difficult  to  play  upon  ?  (171.)  What  is 
the  sonorous  body  in  the  case  of  a  pipe  ?  How  thrown  into  vibration  ?  What  is  a 
mouth-piece?  How  many  forms?  (172.)  What  are  the  characteristics  of  pipes 
with  fixed  mouth-pieces? 


MUSICAL     SOUNDS. 


177 


length  compared  with  their  cross  section.  To  this  class 
belong  the  flute,  the  organ  pipe,  and  the  like.  Some  of  the 
forms  given  to  pipes  of  this  class  are  shown  in  Figs.  116, 
117,  118,  119,  and  120. 

Fig.    116   represents   a   rectangular   pipe  of  wood,  and  Fig.   117 
-hows  the  form  of  its  longitudinal  section.     P  represents  the  tube 


Fig.  118.    Fig.  119. 


Fig.  120. 


through  which  air  is  forced  into  it.  The  air  passes  through  a  narrow 
opening,  i,  called  the  vent.  Opposite  the  vent  is  an  opening  in  the 
side  of  the  pipe,  called  the  mouth.  The  upper  border,  a,  of  the 
mouth,  is  bevelled,  and  is  called  the  upper  lip,  the  lower  border  is  not 
bevelled,  and  is  called  the  lower  Jip. 


Describe  the  month-piece,      The  v?nt.     The  mout'i.     The  lips. 


ITS  POPULAR     PllYSlCa. 

The  current  of  air  forced  through  the  vent  strikes  against  tlie 
upper  lip.  is  compressed,  and  by  its  elasticity,  reacts  upon  the  enter 
ing  current,  and  for  an  instant  arrests  it.  This  stoppage  is  only  for 
an  instant,  for  the  compressed  air  finds  an  outlet  through  the  mouth, 
again  permitting  the  flow.  No  sooner  has  the  flow  commenced  than 
it  is  a  second  time  arrested  as  before,  again  to  be  resumed,  and 
so  on. 

This  continued  arrest  and  release  of  the  current  gives  rise  to  a 
succession  of  vibrations,  which  are  propagated  through  the  tube, 
causing  alternate  and  rapid  condensations  and  rarefactions,  which 
result  in  a  continuous  sound.  The  vibrations  arc  the  more  rapid  as 
the  current  introduced  is  stronger,  and  as  the  upper  lip  approaches 
nearer  the  vent 

Fig.  118  represents  a  second  form  of  organ  pipe,  which  is  shown 
in  section  in  Fig.  1 19.  This  is  but  a  modification  of  the  pipe  already 
explained.  The  letters  indicate  the  same  parts  as  in  the  preceding 
figures. 

Fig.  120  represents  the  form  of  the  mouth-piece  of  the  flageolet, 
and  it  will  be  seen  that  it  bears  a  close  resemblance  to  the  pipes 
already  explained. 

In  the  flute,  an  opening  is  made  in  the  side  of  the  pipe,  and  the 
arrest  and  flow  of  the  current  are  effected  by  the  arrangement  of  the 
lips  of  the  player. 

Reed  Pipes. 

173.  In  REED  PIPES  the  mouth-piece  is  provided  with 
a  vibrating  tongue,  called  a  Reed,  by  means  of  which  the 
air  is  put  in  vibration.  To  this  class  belong  the  clarionet, 
the  hautboy,  and  the  like.  The  reed  may  be  so  arranged 
as  to  beat  against  the  sides  of  the  opening,  or  it  may  play 
freely  through  the  opening  in  the  tube. 

Figs.  121  and  122  show  the  arrangement  of  a  reed  of  the  first  kind. 
A  piece  of  metal,  a.  shaped  like  a  spoon,  is  fitted  with  an  elastic 
tongue.  /,  which  can  completely  close  the  opening.  A  piece  of 

Explain  the  action  in  detail.  How  is  the  mouth-piece  in  fie  flute t  (173.> 
What  is  a  reed  ?  What  are  some  of  the  reed  Instruments?  Explain  the  arrange' 
went  of  a  reed  of  the  first  kind. 


MUSICAL    SOUNDS. 


170 


metal,  r,  which  may  be  elevated  or  depressed  by  a  a  rod,  b.  serves 
to  lengthen  or  shorten  the  vibrating  part  of  the  reed.  This  arrange 
ment  enables  us  to  diminish  or  increase  the  rapidity  of  vibration  at 
pleasure. 

The  mouth-piece,  as  described,  connects  with  the  tube  T,  and  is 
set  in  a  rectangular  box,  JOT,  which  is  in  communication  with  a 
bellows,  from  which  the  wind  is  supplied.  For  the  purpose  of  class 
demonstration,  the  upper  part  of  the  tube  KN,  has  glass  windows 
on  three  sides  to  show  the  motion  of  the  reed. 


Fig.  121. 


Fig.  122. 


Fig.  123. 


When  a  current  of  air  is  forced  into  the  tube  KN.  the  reed  is  set 
in  rapid  vibration,  causing  a  succession  of  rarefactions  and  conden 
sations  in  the  air  of  the  pipe  T,  and  causing  it  to  emit  a  sound.  The 
air  entering  the  tube  KN.  first  closes  the  opening  by  pressing  the 


action. 


180 


POPULAR     PHYSICS. 


reed  against  it ;  the  reed  then  recoils  by  virtue  of  its  elasticity,  per 
mitting  a  portion  of  condensed  air  to  enter  the  pipe,  when  the  reed 
is  again  pressed  against  the  opening,  and  so  on  as  long  as  the  cur 
rent  of  air  is  kept  up.  It  is  evident,  that  the  rapidity  of  vibration 
will  be  increased  by  increasing  the  tension  of  the  air  from  the 
bellows,  and  also  by  shortening  the  vibrating  part  of  the  reed. 

Fig.  123  shows  the  arrangement  of  the  free  reed.  The  vibrating 
plate,  /.  is  placed  so  as  to  pass  backwards  through  an  opening  in  the 
side  of  the  tube  ca.  alternately  closing  and  opening  a  communication 
between  the  tube  and  the  air  from  the  bellows.  The  regulator,  r,  is 
entirely  similar  to  that  shown  in  Figs.  121  and  122,  as  are  the 
remaining  parts  of  the  arrangement.  The  explanation  of  the  action 
of  this  species  of  reed  is  entirely  similar  to  that  already  described. 


Fig.  124. 


Explain  the  arrangement  of  the  Free  Reed.     Whfit  i«  it<  mode  of  action  f 


MUSICAL     SOUNDS. 


181 


The  Bellows. 

174.  Fig.  124  represents  one  form  of  the  Bellows,  used 
in  causing  pipes  to  sound.     It  is  worked  by  a  lever.     The 
air  enters  a  valve,  S,  through  which  it  passes  to  a  leathern 
reservoir,  fi.     The  top  of  the' reservoir  is  weighted  so  as  to 
force  the  air  into  a  box,  from  which  it  is  admitted  to  the 
pipes  by  means  of  valves,  which  are  opened  and  shut  at  the 
will  of  the  player. 

Wind  Instruments. 

175.  WIND    INSTRUMENTS   of  music   consist   of  pipes, 
either  straight  or  curved,  which  are  made  to  sound  by  a 
current  of  air  properly  directed. 

In  some,  the  current  of  air  is  directed  by  the  mouth  upon 
on  opening  made  in  the  side,  as  in  the 
flute.  In  others,  the  current  of  air  is  made 
to  enter  through  a  mouth-piece,  as  in 
the  flageolet.  In  others,  a  reed  is  used, 
as  in  the  clarionet.  In  the  organ,  there  is 
a  collection  of  tubes,  similar  to  those  shown  • 
in  Figs.  116  and  118.  In  some  instru 
ments,  as  the  trumpet  and  the  horn,  a 
conical  mouth-piece  is  used,  of  the  form 
shown  in  Fig.  125,  within  which  the  lips 
of  the  musician  vibrate  in  place  of  the 
reed.  The  rapidity  of  vibration  can  be 
regulated  at  will. 


n.  125. 


(  1 74.)  Describe  the  Bellows  used  with  wind  instruments.  (175)  What  are  Wind 
Instruments  ?    Explain  their  different  varieties. 


CHAPTER  V. 

HEAT. 
I. — GENERAL      PROPERTIES      OF      HEAT 

Definition  of  Heat. 

176.  HEAT  is  the  physical  agent  that  produces  the  sen 
sation  we  call  warmth ;  the  term  heat  is  also  applied  to  the 
sensation  itself.     Cold  is  a  negative  term  used  to  express 
the  absence  of  heat. 

Theories  of  Heat. 

177.  Two  theories  have  been  advanced  to  explain  the 
phenomena  of  heat:  the  emission  theory  and  the  undula- 
tory  theory. 

According  to  the  emission  theory,  heat  is  a  fluid,  destitute  of 
weight  and  capable  of  passing  from  one  body  to  another  with  great, 
velocity.  Its  particles  repel  each  other,  but  are  attracted  by  "the  par 
ticles  of  all  other  bodies.  A  body  becomes  heated  by  receiving  more 
of  this  fluid  than  it  gives  out ;  it  becomes  cooled  by  giving  out  more 
than  it  receives. 

According  to  the  undulatory  theory,  the  heat  of  a  body  is  caused 
by  a  rapid  vibration  of  its  molecules;  this  motion  may  be  trans 
mitted  from  one  body  to  another  through  an  elastic  medium  called 
ether,  in  the  same  way  that  sound  is  transmitted  through  the  air. 
According  to  this  view  heat  is  a  mode  of  motion,  and  those  bodies 
are  hottest  whose  molecules  vibrate  with  greatest  velocity,  and 
through  the  greatest  amplitudes. 

(176.)  What  is  heat?  Cold?  (177.)  What  two  theories  of  heat  have  been 
advanced  ?  Explain  the  emission  theory.  The  nndnlatory  theory.  • 


GENERAL    PROPERTIES    OF    HEAT.  183 

The  undulatory  theory  is  the  one  generally  adopted  by  physicists ; 
it  affords  a  better  explanation  of  the  phenomena  and  at  the  same 
time  serves  to  show  the  intimate  relation  between  heat  and  light. 
In  what  follows  the  phenomena-  will  be  explained,  as  far  as  possible, 
independently  of  both  theories. 

General  Effects  of  Heat. 

1'7§.  Heat,  accumulating  in  bodies,  penetrates  into  their 
substance,  and  acting  upon  their  ultimate  molecules,  gives 
rise  to  repellent  forces  which  counteract  those  of  cohesion. 
Hence,  the  most  noticeable  phenomenon  of  heat  is,  that  it 
causes  bodies  to  expand.  If  applied  in  sufficient  quantity, 
the  particles  of  solids  are  so  far  repelled,  as  to  move  freely 
amongst  each  other,  becoming  liquid  /  or  if  still  greater 
quantities  of  heat  are  applied,  the  body  passes  into  a  state 
of  vapor.  When  heat  is  abstracted  from  a  vapor,  it  returns 
to  a  liquid  state,  and  if  still  more  heat  be  abstracted,  it 
becomes  solid,  and  if  the  process  be  continued,  the  solid 
goes  on  contracting  under  the  influence  of  the  molecular 
forces. 

Hence  we  say,  that  heat  dilates  bodies,  and  cold  contracts 
them.  Heat  also  converts  solids  into  liquids,  liquids  into 
vapors,  and  acting  upon  gases  and  vapors,  causes  them  to 
expand. 

Expansion  of  Bodies  by  Heat. 

179.  All  bodies  are  expanded  by  heat,  but  in  very 
different  degrees.  The  most  dilatable  bodies  are  gases, 
then  vapors,  then  liquids,  and  finally  solids.  In  fluids  we 
regard  only  increase  of  volume,  but  in  solids  we  distinguish 
two  kinds  of  expansion,  linear  expansion,  that  is,  expansion 
in  length,  and  expansion  of  volume 

(178.)  Describe  the  general  effects  of  heat  on  solids.  On  liquids.  What  effect 
has  cold  on  vapors?  On  liquids?  (  179.)  What  holies  are  most  dilatable?  Tha 
least  dilatable  ?  What  is  linear  expansion  ?  Expansion  of  volume  ? 


184 


POPULAR     PHYSICS. 


Fig.  126  represents  the  method  of  showing  and  measuring  the 
linear  expansion  of  the  metals.  A  rod  of  metal,  A^  passes  through 
two  metallic  supports,  being  made  fast  at  one  extremity  by  a  clamp- 
screw.  B.  and  being  free  to  expand  at  the  other  extremity.  The 
free  end  abuts  against  the  short  end,  C,  of  a  lever,  the  long  end.  D, 
of  which  plays  in  front  of  a  graduated  arc. 


Fig.  126. 

When  the  rod   is   heated,  by  placing  fire  beneath  it,  as  shown  in 
the  figure,  the  rod  A  expands,  and   the  expansion  is  shown  by  the 


i-   127 


the  linear  expansion  of  metals  sh 


GENERAL  PROPERTIES  OF  HEAT. 


185 


motion  of  the  index,  D.  When  the  rod,  A.  is  of  steel,  copper,  silver, 
&c..  the  amount  of  expansion  varies,  as  is  shown  by  the  different 
amounts  of  displacement  of  the  index.  Brass,  for  example,  expands 
more,  for  the  same  amount  of  heat,  than  iron  or  steel. 

Fig.  127  shows  the  method  of  demonstrating  that  bodies  undergo 
an  expansion  in  volume  when  heated.  A  ring.  A,  is  constructed  so 
that  a  ball,  B.  passes  freely  through  it  when  cold.  If  the  ball  be 
heated  in  a  furnace,  it  will  no  longer  pass  through  the  ring,  but  if 
allowed  to  cool,  it  again  falls  through  the  ring.  The  method  of 
making  the  experiment  is  fully  shown  in  the  figure. 

Liquids  and  gases  being  more  expansible  than  solids,  their  expan 
sion  is  more  easily  shown  by  ex 
periment.  For  liquids,  we  take  a 
hollow  glass  sphere,  terminating  in 
a  narrow  tube,  open  at  the  top,  and 
fill  the  globe  and  a  portion  of  the 
stem  with  some  fluid,  like  mercury, 
as  shown  in  Fig.  128.  If  heat  be 
applied  to  the  globe,  the  liquid  will 
rise  in  the  stem  from  a  towards  b. 
indicating  an  increase  of  volume  : 
and  if  sufficient  heat  be  applied,  the 
liquid  will  fill  the  stem,  and  will 
ultimately  be  converted  into  vapor. 
If  the  liquid  is  allowed  to  cool,  it 
again  returns  to  its  original  volume. 

An  analogous  experiment  shows 
the  expansion  of  gases  and  vapors. 
A  bulb  of  glass  is  provided  with  a 
long  and  fine  tube  of  the  same  ma 
terial,  which  is  bent  twice  upon 
itself,  as  shown  in  Fig.  129.  An 
index  of  mercury  is  introduced  into 
the  stem  in  the  following  manner. 
The  bulb  is  heated,  and  a  portion  of 
the  air  which  it  contains  is  driven 


Fig.  128 


Fig.  129. 


How  is  expansion  in  volume  shown  t    How  is  the  expansion  of  liquids  shown  f 
0' gases  f 


186  POPULAR     PHYSICS. 

out,  when  a  drop  of  mercury  is  poured  into  the  funnel,  a.  If  the 
instrument  is  allowed  to  cool,  the  air  in  the  bulb  contracts,  and  the 
pressure  of  the  atmosphere  drives  the  drop  of  mercury  along  the 
tube  to  some  position,  m. 

The  instrument  having  been  prepared  in  this  manner,  if  the  bulb 
is  held  in  the  hand  for  a  few  minutes,  the  air  becomes  heated  and 
expands,  the  expansion  being  indicated  by  the  index  moving  to  some 
hew  position,  as  n.  If  allowed  to  cool,  the  index  returns  to  m. 

From  what  precedes,  we  infer  that  heat  expands  all 
bodies,  and  that  cold  contracts  them.  There  are  apparent 
exceptions  to  this  law,  but  they  are  only  apparent.  Thus, 
bodies  capable  of  absorbing  water,  like  paper,  wood,  clay, 
and  the  like,  contract  on  being  heated.  This  contraction 
is  only  apparent ;  it  arises  from  the  water  which  they  con 
tain  being  vaporized  and  driven  off,  which  produces  an 
apparent  diminution  of  volume  ;  after  they  are  thoroughly 
dried,  they  follow  the  general  law. 

The  property  just  explained  is  used  for  bending  absorbent 
bodies.  To  effect  this  they  arc  heated  on  one  side  only,  which 
drives  out  the  water  from  that  side,  and  causes  them  to  bend  in 
that  direction.  It  is  this  principle  that  causes  wooden  articles  to 
warp,  and  therefore  demands  that  articles  of  furniture,  and  wooden 
parts  of  buildings,  be  coated  with  oils,  paints,  or  varnishes,  to  pre 
vent  the  absorption  of  water. 

The  principle  of  expansion  and  contraction  is  often  utilized  in 
the  arts. 

A  familiar  example,  is  the  process  of  setting  the  tire  of  a  wagon- 
wheel.  The  tire  is  made  a  little  smaller  than  the  outer  periphery 
of  the  wooden  part  of  the  wheel.  It  is  then  heated,  and  placed 
around  the  wheel;  on  cooling,  it  contracts  powerfully,  and  draws 
the  felloes  firmly  together.  The  same  principle  has  been  applied 


What  is  the  general  conclusion  -with  respect  to  the  action  of  heat  and  cold  ? 
Explain  the  apparent  exceptions  to  the  law.  Explain  the  process  of  warping 
Are  the  pri"dples  o?  contraction  and  expansion  iiUUsedf  Explainthe  operation 
of  setting  a  tire.  Of  drawing  walls  together. 


THERMOMETERS.  187 

in  bringing  the  walls  of  a  building  back  to  their  original  position 
after  they  had  begun  to  separate  from  each  other. 

Sensible   and  Latent  Heat.  —  Temperature. 

ISO.  Heat  may  act  on  a  body  in  two  ways.  First,  it 
may  act  to  increase  the  warmth  of  the  body;  in  this  case 
it  is  said  to  be  sensible.  Secondly,  ifc  may  be  absorbed  and 
act  solely  to  produce  a  change  of  state  of  the  body,  without 
becoming  manifest  to  the  senses ;  in  this  case  it  is  said  to 
be  latent.  Thus,  when  ice  melts,  it  absorbs  an  immense 
amount  of  heat  without  appearing  to  become  any  warmer; 
this  heat  acts  to  change  the  body  from  a  solid  to  a  liquid  state. 

The  temperature  of  a  body  is  the  amount  of  its  sensible 
heat. 

II.  —  THERMOMETERS. 

The    Thermometer. 

181.  A  THERMOMETER  is  an  instrument  for  measuring 
temperatures. 

The  thermometer  depends  upon  the  principle  that  bodies 
expand  when  heated,  and  contract  when  cooled.  Ther 
mometers  have  been  constructed  of  a  great  variety  of 
materials.  For  common  purposes,  the  mercurial  thermome 
ter  is  preferred,  on  account  of  the  uniformity  with  which 
both  mercury  and  glass  expand  when  heated. 

The  mercurial  thermometer  consists  of  a  bulb  of  glass,  at 
the  upper  extremity  of  which  is  a  narrow  tube  of  uniform 
bore,  hermetically  sealed  at  its  upper  end.  The  bulb  and  a 
part  of  the  tube  are  filled  with  mercury,  and  the  whole  is 
attached  to  a  frame  on  which  is  a  scale  for  measuring  the 
rise  and  fall  of  the  mercury  in  the  tube. 

(180.)  What  is  sensible  heat?  Latentheat?  Temperature?  (181.)  What  is  a 
Thermometer?  On  what  principle  does  it  depend?  What  is  the  best  thermometer 
for  common  use  ?  Describe  a  mercurial  thermometer. 


188 


POPULAR     PHYSICS. 


Method  of  making  a  Thermometer. 

182.  A  capillary  tube  of  glass  is  provided, 
of  uniform  bore,  upon  one  end  of  which  a  bulb 
is  blown,  and  upon  the  other  a  funnel,  as  shown 
in  Fig.  130. 

The  funnel  is  nearly  filled  with  mercury, 
which  is  at  first  prevented  from  penetrating  into 
the  bulb  by  the  resistance  of  the  air  and  the 
smallness  of  the  tube.  The  bulb  is  therefore 
heated,  when  the  air  within  expands,  and  a  por 
tion  escapes  in  bubbles  through  the  mercury. 
On  cooling,  the  pressure  of  the  external  atmos 
phere  forces  a  quantity  of  mercury  through  the 
tube  into  the  bulb.  By  repeating  this  operation 
a  few  times,  the  bulb  and  a  portion  of  the  tube 
are  filled  with  mercury. 

The  whole  is  then  heated  till  the  mercury 
boils,  thus  filling  the  tube,  when  the  funnel  is 
melted  off  and  the  tube  hermetically  sealed  by 
means  of  a  jet  of  flame  urged  by  a  blow-pipe. 
On  cooling,  the  mercury  descends  to  some  point 
of  the  tube,  as  shown  in  Fig.  131.  leaving  a 
vacuum  at  the  upper  end.  It  only  remains 
to  graduate  it,  and  attach  a  suitable  scale. 


Figs  130.     131. 


Method  of  Graduation. 

183.  Two  points  of  the  stem  are  first  determined,  the  freezing 
and  the  boiling  point.  These  are  determined  on  the  principle  that 
the  temperatures  at  \vhich  distilled  water  freezes  and  boils,  are 
always  the  same,  that  is.  when  these  changes  of  state  take  place 
under  equal  atmospheric  pressures. 

The  instrument  is  first  plunged  into  a  bath  of  melting  ice,  as 
shown  in  Fig.  132,  and  is  allowed  to  remain  until  it  takes  the 


(182-)  Describe  the  process  of  filling  a  thermometer  with  mercury.  How  is 
ihe  tube  sealed  ?  (183.)  On  what  principle  are  the  freezing  and  boiling  points 
dttermintd  ' 


THERMOMETERS. 


189 


temperature  of  the  mixture,  say  twenty  or  thirty  minutes.     A  slight 
scratch  is  then  made  on  the  stem  at  the  upper  surface  of  the  mer 
cury,  and  this  constitutes  the 
freezing  point. 

The    instrument   is   next 
plunged  into  a  bath  of  dis 
tilled  water,  in  a  state  of 
ebullition,  care  being  taken 
to  surround  it  with  steam 
by  means  of  an  apparatus 
like  that  shown  in  Fig.  133. 
After  the  mercury  ceases  to 
rise  in  the  tube,  which  will 
be   in   a   few  minutes,  the 
level  of  its  upper  surface  is 
marked  on  the  stem,  by  a 
scratch,  as  before,  and  this 
constitutes  the  boiling  point. 
The   space  between   the 
boiling  and  freezing  points  is 
then  divided  into  a  certain 
number  of  equal  parts,  and 
the  graduation  is  continued 
above  and  below  as  far  as 
may  be  desired.     These  di 
visions    may  be    scratched 
upon  the  glass  with  a  dia 
mond,  or,  as  is  usually  done, 
they  may  be  made  on  a  strip  Fig.  132. 

of  metal,  which  is  attached 

to  the  frame.     The  divisions  are  numbered  according  to  the  kind 
of  scale  adopted. 

Thermometer  Scales. 

184.     Three  principal   scales  are  used :  the  Centigrade 
scale,  in  which  the  space  between  the  freezing  and  boiling 


How  is  the  freezing  point  determined  f    The  boiling  point  f    How  is  the  inter 
mediate  space  divided  t    (184.)  What  are  the  three  principal  scales  used  ? 


190 


POPULAR     PHYSICS. 


points  is   divided  into    100   equal    parts,  called   degrees  • 

Reaumur  }s  scale,  in  which  the  same  space  is  divided  into  80 

equal  parts,  called  degrees  y 

and  Fahrenheit's   scale,  in 

which  this  space  is  divided 

into   180   equal    parts,   also 

called  degrees. 

In  the  centigrade  scale, 
the  freezing  point  is  marked 
0,  and  the  degrees  are  num 
bered  both  up  and  down, 
the  former  numbers  beinc: 

O 

considered  positive,  and 
designated  by  the  sign  +  , 
whilst  the  latter  are  con 
sidered  negative,  and  desig 
nated  by  the  sign  — .  Of 
course  the  boiling  point  is 
marked  100°. 

Fig.  134  represents  a  ther 
mometer  mounted  and  gradu 
ated  according  to  the  centigrade 
scale.  In  it  the  mercury  indi 
cates  30°  C. 

In  Reaumur's  scale,  the 
freezing  point  is  marked  0, 
and  the  boiling  point  80°. 
The  degrees  below  freezing 
are  marked  as  in  the  centi 
grade  scale. 

In  Fahrenheit's  scale,  which  is  the  one  most  used  in  the 
United  States,  the  0  point  is  taken  32°  below  the  freezing 


Where  is  the  0  point  of  the  centigrade  scale?  Explain  the  signs  +  and  -,  What 
is  the  boiling  point  marked  ?  Where  is  the  0  of  the  Keaumur  scale  ?  The  boiJin» 
point  ?  Where  is  the  0  of  Fahrenheit's  scale  ? 


THERMOMETERS. 


191 


point,  and  the  divisions  are  numbered  from  this  point  both 
up  and  down.     The  boiling  point  of  distilled  water  is,  212°  F. 


Conversion  of  Centigrade 
and  Reaumur's  Degrees 
into  Fahrenheit's. 


A  degree  on  the 
cenii  grade  scale  is  equal  to 
one  and  eight  tenths  of  a 
degree  on  the  Fahrenheit 
scale,  and  one  on  Reaumur's 
scale  is  equal  to  two  and 
a  quarter  on  Fahrenheit's. 
Hence,  to  convert  the  reading 
on  a  centigrade  to  an  equiva 
lent  one  on  Fahrenheit's  scale, 
multiply  it  by  1.8  and  add  to 
the  result  32°.  Thus,  a  read 
ing  of  25°  centigrade,  is 
equivalent  to  25°  x  1.8  4-  32°, 
or  77°  F.  To  convert  a  read 
ing  on  Reaumur's  scale  to  an 
equivalent  one  on  Fahren 
heit's,  multiply  by  2±,  and  to 
the  result  add  32°.  Thus,  a 
reading  of  24°  Reaumur  is 
equivalent  to  24°  X  2i  +  32°, 
or  86°  F. 

By  reversing  the  above 
processes,  readings  on  Fah 
renheit's  scale  may  be  con 
verted  into  equivalent  ones  on 
the  centigrade  or  Reaumur's 
scale. 


Fig.  134. 


The  boiling  point?    (185.)  Explain  the  method  of  converting  reading*  from 
one  scale  tj  another. 


192  POPULAR    PHYSICS. 


Alcohol   Thermometers. 

186.  An  ALCOHOL  THERMOMETER  is  similar  to  a  mercu 
rial  one  in  all  respects,  except  that  alcohol,  tinged  red,  is 
used  in  place  of  the  mercury. 

Because  alcohol  does  not  expand  regularly  with  a  regular  increase 
of  temperature,  the  alcohol  thermometer  has  to  be  graduated  by 
experiment,  comparing  it  degree  by  degree  with  a  standard  mercurial 
thermometer. 

An  alcohol  thermometer  is  more  easily  filled  than  a  mercurial  one, 
no  funnel  being  required.  The  bulb  is  heated  until  a  portion  of  the 
contained  air  is  driven  off,  and  then  the  open  end  of  the  tube  is 
plunged  into  a  vessel  of  alcohol.  As  the  air  in  the  bulb  cools,  the 
pressure  of  the  external  atmosphere  forces  a  portion  of  alcohol  up 
into  the  bulb.  If  this  be  boiled,  the  vapor  of  alcohol  will  expel  the 
remainder  of  the  air,  and  by  dipping  the  open  end  of  the  tube  into 
the  alcohol  once  more,  the  bulb  will  be  completely  filled,  when  it 
again  becomes  cool.  The  instrument  is  then  treated  like  the  mercu 
rial  thermometer. 

Relative  advantages  of  Mercurial  and  Alcohol  Thermometers. 

187.  For  ordinary  purposes,  the  mercurial  thermometer  is  to 
be  preferred,  on  account  of  the  uniformity  with  which  the  mercury 
expands  with  a  uniform  increase  of  temperature.  But  mercury  con 
geals  at  39°  below  0  of  the  Fahrenheit  scale,  and  where  a  lower 
temperature  than  this  is  to  be  observed,  it  becomes  absolutely 
necessary  to  employ  the  spirit  thermometer.  In  the  severe  cold  of 
the  polar  regions,  mercury  often  congeals,  but  no  degree  of  cold  has 
yet  been  obtained  that  will  congeal  absolute  alcohol. 

For  high  temperatures,  mercury  only  is  capable  of  being  used ; 
this  liquid  does  not  boil  till  raised  to  662°  F.,  whilst  alcohol  boils  at 
174°  F.  The  latter  liquid  can  not,  therefore,  be  used  to  observe  tem- 


(186.)  How  does  the  alcohol  differ  from  the  mercurial  thermometer?  Row  is 
the  alcohol  thermometer  graduated  f  Why  T  How  is  it  filled  f  (1ST.)  When  is 
the  alcohol  thermometer  preferable  to  the  mercurial  one?  When  must  the  latter 
be  used? 


THERMOMETERS.  193 

peratures  higner  than  174°  F.,  nor  can  it  be  relied   upon   even  for 
temperatures  considerably  lower  than  this. 

It  is  to  be  observed,  that  mercury  can  not  be  relied  upon  for  tem 
peratures  lower  than  32°  below  0,  on  account  of  irregularities  in  its 
rate  of  contraction  below  that  limit. 


Rules  for  using  a  Thermometer. 

188.  Before  noting  the  height  of  the  mercurial  column, 
the  instrument  should  be  allowed  to  acquire  the  temperature 
of  the  medium  in  which  it  is  placed.     This,  in  general,  will 
require  some  minutes. 

In  determining  the  temperature  of  a  room,  the  thermom 
eter  should  not  be  hung  against  the  walls,  but  should  be 
freely  suspended,  so  as  to  take  the  temperature  of  the 
atmosphere.  When  hung  against  a  wall,  especially  an  outer 
wall,  an  error  of  several  degrees  may  result.  In  like  man 
ner,  if  hung  against  a  wall  containing  a  flue,  or  adjoining 
another  room  of  different  temperature,  a  similar  error  of 
several  degrees  might  result. 

To  determine  the  temperature  of  the  atmosphere,  the 
thermometer  should  be  freely  suspended  in  the  air  at  some 
distance  from  any  building  or  tree.  It  should  be  sheltered 
from  the  direct  action  of  the  sun's  rays,  as  well  as  from  the 
influence  of  reflecting  substances.  Furthermore,  it  should 
be  protected  from  winds  and  currents  of  air. 

The  Differential   Thermometers. 

189.  A  DIFFERENTIAL  THERMOMETER  is  a  thermometer 
contrived  to  show  the  difference  of  temperature  between 
two  places  near  each  other.     The  two  principal  forms  of  the 
differential  thermometer  are  RUMFORD'S  and  LESLIE'S. 


When  can  the  former  only  be  used?  (188.)  What  precautions  are  to  be  taken 
in  noting  the  temperature  of  a  room?  Why?  In  noting  the  temperature  of  the 
atmosphere  ?  (1 89.)  What  is  a  Differential  Thermometer  ?  What  are  its  frvo  forms  ? 


194 


POPULAR    PHYSICS. 

Rumford's  Differential  Thermometer. 


19O.  RUMFORD'S  DIFFERENTIAL  THERMOMETER  is  repre 
sented  in  Fig.  135. 

It  consists  of  two  bulbs  of  thin  glass.  A  and  B,  connected 
by  a  fine  tube  bent  twice  at  right  angles,  as  shown  in  the 


Fi-   135. 

figure.  The  whole  apparatus  is  attached  to  a  suitable 
frame,  which  supports  a  scale  parallel  to  the  horizontal 
branch  of  the  connecting  tube.  The  0  of  the  scale  is  at  its 
middle  point,  and  the  graduation  is  continued  from  it  in 
both  directions.  The  bulbs  and  a  large  part  of  the  connect 
ing  tube  are  filled  with  air ;  there  is,  however,  in  the  tube 
a  small  drop  of  fluid  which  separates  the  air  in  the  two 
extremities. 

The  instrument  is  so  constructed  that  the  index,  n,  is  at 
the  0  of  the  scale  when  the  temperature  of  the  two  bulbs  is 

(19O)  Describe  RUUFOBD'S  form.    Explain  the  scale.     Explain  its  action.     How 
is  the  scale  graduated  ? 


THERMOMETERS. 


195 


the  same.  When  one  of  the  bulbs  is  heated  more  than  the 
other,  the  air  in  it  expands  and  drives  the  index  towards 
the  other,  until  the  tensions  of  the  air  in  the  two  bulbs 
exactly  balance  each  other. 

The  scale   is   divided   by  experiment   by  the  aid   of  a 
standard  mercurial  thermometer. 


Leslie's  Differential  Thermometer. 

191.  LESLIE'S  DIFFERENTIAL 
THERMOMETER  is  shown  in  Fig. 
136.  It  differs  from  RUMFORD'S, 
in  having  the  bulbs  smaller,  and 
in  containing  a  longer  column  of 
liquid  in  the  tube.  The  scales 
are  placed  by  the  sides  of  the 
vertical  portions  of  the  tube, 
having  their  0  points  at  the  mid 
dle.  There  is,  then,  a  double 
scale.  The  method  of  graduating 
and  using  this  thermometer  is 
the  same  as  that  described  in 
the  last  article. 

Pyrometer. 


Fig.  136. 


192.  A  PYROMETER  is  an  instrument  for  measuring 
higher  temperatures  than  can  be  observed  by  means  of  the 
mercurial  thermometer. 

The  most  important  pyrometers  are  those  of  WEDGEWOOD 
and  BROGNIAHT.  The  former  is  founded  on  the  diminution 
of  the  volume  of  clay  at  high  temperatures,  and  the  latter 
on  the  principle  of  the  expansion  of  metals.  The  indications 
of  these  instruments  are  very  unreliable,  and  it  yet  remains 


(191.)  Describe  LESLIE'S  Differential  Thermometer.  (192.)  What  is  a  Pyro 
meter?  What  are  the  most  important  ones  ?  What  is  the  principle  of  each  ?  Are 
they  reliable  ? 


196  POPU-LAK,    PHYSICS. 

to  discover  some  accurate  method  of  measuring  tempera 
tures  higher  than  000°  F. 

III.  — 11  A  D  I  A  T  I  O  N    OF     HEAT. 

Propagation  oi  Heat. 

193.  The  ethereal  medium  that  transmits  heat  extends 
through  space,  and  is  almost  perfectly  elastic.      It  pene 
trates  all  bodies  and  occupies  the  intervals  between  their 
molecules.      The  heat  vibrations  of  bodies  are  thus  im 
parted  to  the  surrounding-  ether,  and  by  it  are  propagated 
outward  in  spherical  waves  similar  to  sound-waves  in  air. 
Heat  propagated  in  this  way  is  called  radiant  heat.     A  line 
perpendicular  to  a  wave  front  is  called  a  ray  of  heat. 

A  ray  of  heat  indicates  a  direction  in  which  heat  is  propagated 
and  along  which  it  produces  its  effect.  In  a  homogeneous  medium 
heat-rays  are  straight  lines  radiating  in  every  direction  from  a  heated 
body.  Rays  of  heat,  like  rays  of  sound,  may  be  refracted  and  reflected. 
Radiant  heat  does  not  impart  warmth  to  the  medium  that  transmits 
it,  but  when  intercepted  by  a  body  the  motion  of  the  particles  of 
ether  is  imparted  to  the  molecules  of  the  body,  and  the  phenomena 
of  heat  are  developed. 

Laws  of  Radiant  Heat. 

194.  The  radiation  of  heat  takes  place  according  to  the 
following  laws : 

1.  Heat  is  radiated  equally  in  all  directions. 

This  law  may  be  verified  by  placing  thermometers  at  equal  dis 
tances  and  in  different  directions  from  a  heated  body. 

2.  Rays  of  heat  are  straight  lines. 

This  law  may  be  verified  by  interposing  a  screen  anywhere  in  a 
right  line  joining  the  heated  body  and  the  thermometer,  when  the 
thermometer  will  cease  to  rise. 

(193.)  How  is  heat  transmitted  through  space  ?  Whfct  is  radiant  heat?  What 
are  rays  of  heat?  (194.)  What  is  the  first  law  of  radiant  heat?  Hoiv  verified? 
What  is  the  second  law  ?  How  verified? 


RADIATION     OF     HEAT.  19 1 

If  a  ray  pass  from  one  medium  to  another,  it  is  bent  from  its  course , 
this  bending  is  called  refraction. 

The  laws  of  refraction  for  heat  are  the  same  as  for  sound. 

1°.  The  plane  of  the  incident  and  refracted  rays  is  normal  to  the  de 
viating  surface  at  the  point  of  incidence, ;  and 

2\  The  sine  of  the  angle  of  incidence  bears  a  constant  ratio  to  the  sine 
of  the  angle  of  refraction. 

3.  The  intensity  of  radiant  heat  varies  directly  as  the 
temperature  of  the  radiating  body,  and  inversely  as  the 
square  of  the  distance  to  which  it  is  transmitted. 

The  first  part  of  this  law  is  verified  by  exposing  one  of  the  bulbs 
of  a  differential  thermometer  to  a  blackened  cubical  box,  filled  with 
hot  water,  the  other  bulb  being  protected  by  a  screen.  If  the  water 
is  in  the  first  instance  of  a  given  temperature,  and  then  falls  to  a 
half,  or  a  third  of  that  temperature,  the  differential  thermometer  will 
manifest  a  half,  or  a  third  of  its  original  indication,  and  so  on  for 
any  temperature. 

The  second  part  of  the  law  may  also  be  verified  by  means  of  the 
differential  thermometer.  In  this  case  the  heated  body  is  kept 
always  at  the  same  temperature,  and  one  bulb  of  the  differential 
thermometer  is  placed  at  different  distances  from  it.  It  will  be 
found  that  at  a  double  distance  the  indication  is  only  a  fourth  of  the 
original  indication,  at  a  triple  distance  only  a  ninth,  and  so  on. 

Exchange  of  Heat  between  bodies. 

1®5.  The  process  of  radiation  of  heat  between  bodies  is 
mutual  and  continuous.  According  to  the  laws  given  in 
the  preceding  article,  those  bodies  which  are  most  heated 
give  off  most  heat;  hence,  the  hottest  bodies  of  a  group 
give  off  more  heat  than  they  receive,  and  the  coldest  ones 
receive  more  than  they  give  off.  The  consequence  is  that 
there  is  a  continual  tendency  towards  equalization  of  tem- 

What  is  the  third  law  ?  How  is  the  first  part  of  the  law  verified?  The  second 
part?  Explain  the  law?  of  refraction  of  heat  rays*.  (195.)  Explain  the  action 
of  radiation  to  produce  uniformity  of  temperature. 


198  POPULAR     PHYSICS. 

perature.  If  all  the  bodies  are  of  the  same  temperature, 
each  will  give  off  as  much  as  it  receives,  and  no  further 
change  of  temperature  can  occur.  The  process  of  radiation, 
however,  goes  on  as  before. 

All  the  bodies  in  a  room,  for  example,  tend  to  come  to  a  uniform 
temperature.  We  say.  tend  to  come  to  a  uniform  temperature,  be 
cause  this  condition  is  never  fully  realized.  Bodies  nearest  the 
walls  are  continually  exchanging  heat  with  the  walls,  and  as  these 
are  in  communication,  either  with  the  outer  air.  or  with  other  rooms, 
their  temperature  will  be  influenced  thereby,  and  will  in  turn  exert 
an  influence  upon  the  remaining  bodies  in  the  room. 


V. — REFLECTION,     ABSORPTION,     EMISSION,     AND    CONDUCTIBILITT. 

Reflection  of  Heat. 

196.  When  a  ray  of  heat  falls  upon  the  surface  of  a 
body,  it  is  divided  into  two  parts,  one  of  which  enters  the 
body  and  is  absorbed,  whilst  the  other  is  deflected  or  bent 
from  its  course.  This  bending  is  called  reflection. 

The  point  at  which  the  bending  takes  place,  is  called  the 
point  of  incidence.  The  ray  before  incidence  is  called  the 
incident  ray  /  nfter  incidence  it  is  called  the  reflected  ray. 
If  a  perpendicular  be  drawn  to  the  surface  at  the  point  of 
incidence,  it  is  said  to  be  normal  to  the  surface  at  that 
point.  The  angle  between  the  incident  ray  and  the  normal 
is  the  angle  of  incidence ;  the  angle  between  the  normal 
and  the  reflected  ray  is  the  angle  of  reflection.  The  plane 
of  the  incident  ray  and  the  normal  is  the  plane  of  incidence ; 
the  plane  of  the  reflected  ray  and  the  normal  is  the  plane 
of  reflection.  These  planes  coincide. 

Illustrate  by  the  example  of  articles  in  a  room.  (196.)  What  is  reflection  of 
heat?  What  is  the  point  of  incidence  ?  The  incident  ray  ?  The  reflected  ray? 
The  plane  of  incidence  ?  The  plane  of  reflection  ?  The  angles  of  incidence  and 
reflection  ? 


EE FLECTION    OF    HEAT. 


199 


Laws  which  govern  the  Reflection  of  Heat. 

197.  The  following  laws,  indicated  by  theory,  have  been 
confirmed  by  experience : 

1.  The  plane  of  the  incident  and  reflected  rays  is  normal 
to  the  reflecting  surface  at  the  point  of  incident. 

2.  Trie  angles  of  incidence  and  reflection  are  equal. 

The  apparatus,  employed  in  establishing  these  laws,  is  shown  in 
Fig.  137.  A  is  a  tin  box  with  its  faces  blackened,  in  which  hot 
water  is  placed.  B  is  a  reflecting  surface,  and  D  is  a  differential 
thermometer.  BC  is  a  normal  to  the  reflecting  surface. 


.-    •  '-.     "*?  '  :^ 

Fig  137. 


The  surface,  A,  radiates  heat  in  all  directions,  but  only  a  single 
ray  is  permitted  to  fall  upon  the  reflector,  B.  the  remainder  being 
intercepted  by  a  screen,  having  a  small  hole  in  it.  By  suitably  ar 
ranging  the  thermometer,  and  other  parts  of  the  apparatus,  it  may 
be  shown  that  the  plane  ABD  is  normal  to  the  reflecting  surface  at 
B,  and  that  the  angles,  ABC  and  CBD,  are  equal  to  each  other. 


(197.)  What  is  the  first  law  of  reflection  ?    The  second  law  ?    Explain  the  ap 
paratus  for  verifying  these  laws.    Explain  the  mode  of  verification. 


200 


POPULAR    PHYSICS. 


Reflection  of  Heat  from  Concave  Mirrors. 

198.  A  CONCAVE  MIRROR  is  a  polished  spherical  or  par 
abolic  surface,  usually  of  metal,  employed  to  concentrate 
rays  of  heat  at  a  single  point.  For  experimental  purposes 
the  parabolic  mirror  is  generally  used. 

It  is  a  property  of  such  mirrors  that  all  rays  which  before 
incidence  are  parallel  to  the  axis,  arc  after  reflection  con 
verged  to  a  single  point,  which  point  is  the  focus  of  the 
mirror.  Conversely,  if -the  rays  proceed  from  the  focus  they 
will  be  reflected  in  lines  parallel  to  the  axis. 

A  and  B,  Fig.  138,  represent  two  parabolic  reflectors, 
having  their  axes  coincident,  and  their  surface  turned  to 
each  other.  In  the  focus,  n,  of  the  mirror,  A,  is  placed  a  ball 


Fi".  138. 


of  hot  iron,  and  in  the  focus,  m,  of  the  mirror,  _/>,  is  placed 
an  inflammable  substance,  as  a  piece  of  phosphorus.     The 


(198.)  What  is  a  Concave  Mirror  ?    What  form  is  used  for  experiment  ?    TTmv 
are  rays  parallel  to  the  axis  reflected  ?     What  is  the  focus  ? 


KEFLECTIOX     OF     HEAT. 


2U1 


heat  radiating  from  the  ball,  is  reflected  from  A,  parallel  to 
the  common  axis  of  the  mirror,  and  falling  upon  7?,  is  again 
reflected  to  the  focus  m ;  the  heat,  concentrated  at  m, 
is  sufficient  to  inflame  the  phosphorus,  even  when  the 
mirrors  are  several  yards  distant  from  each  other.  If  the 
mirror,  A,  alone  is  used,  the  phosphorus  is  not  inflamed. 


Tiio  property  of  parabolic  mirrors,  above  explained,  enables  us  to 
concentrate  the  heat  of  the  sun's  rays.  In  this  case  the  reflector  is 
called  a  burning  mirror.  Fig.  139  shows  the  manner  of  using  a 
burning  mirror.  It  is  placed  so  that  its  axis  is  parallel  to  the  rays 
of  the  sun.  which,  on  falling  upon  it.  are  reflected  to  the  focus,  where 
they  produce  heat  enough  to  set  inflammable  substances  on  fire. 

It  is  said  that  ARCHIMEDES  was  enabled  by  means  of  mirrors  to 


How  are  rays  from  the  focus  reflected  ?    Explain  the  experiment.     What  is  a 
burning  mirror  ?    Explain  its  ti*e. 


202 


POPULAR    PHYSICS. 


set  fire  to  the  Roman  ships  in  the  harbor  of  the  City  of  Syracuse. 
BUFFON  showed  the  possibility  of  .such  an  operation,  by  setting  fire 
to  a  tarred  plank,  by  means  of  burning  mirrors,  at  a  distance  of  more 
than  220  feet 

Reflecting  Power   of    different   substances. 

199.  It  has  been  stated  that  a  ray  of  heat  which  fal  * 
upon  a  body  is  divided  into  two  parts,  one  being  absorbed 
and  the  other  reflected.  The  relative  proportions  between 
these  two  parts  varies  with  the  nature  of  the  substance  and 
the  character  of  the  reflecting  surface. 

Those  bodies  which  reflect  a  large  portion  of  the  incident 
heat,  are  called  good  reflectors ;  those  which  reflect  but 
little  of  the  incident  heat,  are  called  lad  reflectors.  Good 
reflectors  are  bad  absorbers;  and  bad  reflectors  are  good 
absorbers. 

Fig.  140  shows  the  method  of  determining  the  relative 


Fig.  140. 


(199.)     Into  how  many  parts  is  an  incident,  ray  divided?     What  is  a  good  reflec 
tor?     A  bad  reflector?     A  good  nbsorber?     A  b.id  nb^orbe- ? 


ABSORPTION     OF    HEAT.  203 

reflecting  powers  of  different  bodies,  adopted  by  LESLIE. 
He  placed  a  cubical  tin  box  filled  with  water  at  the  boiling 
point,  in  front  of  a  parabolic  reflector.  The  rays  of  heat, 
falling  upon  the  reflector,  are  reflected  and  tend  to  come  to 
a  focus  at  F,  but  by  interposing  a  square  plate  of  some  sub 
stance  between  the  mirror  and  its  focus,  the  rays  are  again 
reflected,  and  come  to  a  focus  as  far  in  front  of  the  plate, 
as  F  is  behind  it.  The  heat  thus  reflected  is  received  upon 
one  bulb  of  a  differential  thermometer,  by  means  of  which 
it  is  measured.  By  interposing  plates  of  different  sub 
stances  in  succession,  their  relative  reflecting  powers  are 
determined. 

In  this  way  LESLIE  showed,  that  polished  brass  possessed 
the  highest  reflecting  power  ;  silver  reflects  only  nine  tenths, 
tin  only  eight  tenths,  and  glass  only  one  tenth  as  much  as 
brass.  Plates  blackened  by  smoke  do  not  reflect  heat 
at  all. 

Absorbing  Power. 

2OO.  In  order  to  determine  the  relative  powers  of  ab 
sorption,  LESLIE  employed  the  apparatus  shown  in  Fig.  141. 

The  source  of  heat  and  the  reflector  remaining  as  before, 
he  placed  the  bulb  of  the  differential  thermometer  in  the 
focus  of  the  reflector,  covering  it  successively  with  layers 
of  the  substance  to  be  experimented  upon.  In  this  way 
he  showed,  that  those  substances  which  reflect  most  heat 
absorb  least,  and  the  reverse. 

When  the  bulb  was  blackened  by  smoke,  the  thermometer 
indicated  the  greatest  change  of  temperature,  and  when 
covered  with  leaves  of  brass,  it  indicated  the  least  change. 


Explain  LESLIE'S  method  of  determining  the  reflecting  power  of  different  bodies. 
What  did  LESLIE  find  to  be  the  best  reflector  ?  The  next  in  order?  What  of  black 
ened  plates  ?  ( 200.)  Explain  LESLIE'S  method  of  determining  the  absorbing  power 
of  bodies.  What  was  the  result  of  his,  experiments  ? 


204- 


POPULAR    PHYSICS. 


Fig.  141. 

Radiating  Power. 

2O1.  The  KADIATING  POWER  of  a  body  is  its  capacity 
to  emit,  or  radiate  the  heat  which  it  contains. 

In  determining  the  radiating  power,  LESLIE  employed 
the  apparatus  shown  in  Fig.  141.  In  this  case,  instead  of 
covering  the  bulb  of  the  thermometer  Avith  layers  of  the 
substances  to  be  experimented  upon,  lie  covered  the  differ 
ent  faces  of  the  cubic  box  with  layers  of  the  different 
substances. 

For  example,  let  one  face  be  made  of  tin,  let  a  second  be 
blackened  by  smoke  or  lamp-black,  let  a  third  be  covered 
by  a  layer  of  paper,  and  a  fourth  by  a  plate  of  glass.  On 
turning  these  different  faces  towards  the  reflector,  the 
thermometer  indicates  different  degrees  of  temperature.  If 
the  blackened  face  be  turned  towards  the  reflector,  the 
thermometer  rises,  showing  that  this  face  is  a  good  ra 
diator  ;  if  the  paper-covered  face  be  next  turned  towards 


( 201 .)  What  is  the  Radiating  Power  of  a  body  ?    Explain  LESLIE'S  method  of  de 
termining  it     Give  an  example  of  his  process. 


EMISSION     OF     HEAT.  205 

the  reflector,  the  thermometer  falls,  showing  that  paper  is  a 
poorer  radiator  than  lamp-black  ;  if  the  glass  covered  face 
be  turned  towards  the  reflector,  the  thermometer  falls  still 
lower,  indicating  that  glass  is  a  poorer  radiator  than  paper ; 
finally,  if  the  tinned  face  is  turned  towards  the  reflector,  the 
thermometer  falls  still  lower,  indicating  the  fact  that  tin  is  a 
poorer  radiator  than  glass. 

LESLIE  found  by  this  course  of  proceeding,  that  the 
radiating  powers  of  bodies  arc  the  same  as  their  absorbing 
powers  ;  that  is,  a  good  radiator  is  also  a  good  absorber,  but 
a  bad  reflector,  and  the  reverse. 

Modifications  of  the  Reflecting  Powers  of  Bodies. 

2O2.  The  principal  causes  that  modify  the  reflecting  and 
absorbing  powers  of  bodies,  are  :  polish,  density,  direction 
of  the  incident  rays,  nature  of  the  source  of  heat,  and 
color. 

Other  things  being  equal,  polished  bodies  are  letter 
reflectors  and  worse  absorbers  than  unpolished  ones. 

Other  things  being  equal,  dense  bodies  are  better  reflectors 
and  worse  absorbers  than  rare  ones. 

Other  things  being  equal,  the  nearer  the  incident  ray 
approaches  the  normal,  the  less  will  be  the  portion  reflected 
and  the  greater  the  portion  absorbed. 

The  nature  of  the  source  of  heat  sometimes  modifies  the 
reflecting  and  absorbing  powers.  Thus,  if  a  body  is  painted 
with  white  lead,  it  absorbs  more  heat  from  a  cubical  box  of 
boiling  water,  than  though  the  same  heat  were  emitted  by 
a  lamp.  But  if  a  body  is  painted  with  lamp-black,  the 
amount  absorbed  is  the  same,  whatever  may  be  its  source. 


What  relation  did  ho  find  between  the  radiating  and  ab? orbing  powers  of  bodies  ? 
(202.)  What  causes  modify  the  reflecting  and  absorbing  powers  of  bodies?  Effect 
of  polish  ?  Of  density  ?  Of  direction  of  rays  ?  Of  the  source  of  hc-at  ? 


206  POPULAR     PHYSICS. 

Other  things  being  equal,  light-colored  bodies  absorb  less 
and  reflect  more  heat  than  dark-colored  ones.  White 
bodies  are  the  best  reflectors,  black  ones  the  worst.  White 
bodies  are  the  worst  absorbers,  and  black  ones  the  best. 


Applications   of  the  preceding  principles. 

2O3.  Articles  of  clothing  arc  intended  to  preserve  uniformity  of 
temperature  in  the  human  body  by  excluding  the  too  violent  heats 
of  summer,  and  by  preventing  too  rapid  radiation  of  animal  heat  in 
winter. 

Loose  substances,  like  woollens  and  furs,  are  bad  radiators,  and 
therefore  are  suitable  for  winter  clothing.  Compact  substances,  like 
linens  and  cottons,  are  good  reflectors,  and  therefore  arc  suitable  IV r 
summer  clothing.  As  far  as  color  -is  concerned,  white  is  best 
adapted  to  both  seasons,  because  white  bodies  are  at  once  better 
reflectors  and  wo;-se  radiators,  than  those  of  dark  colors. 

The  animals  of  the  polar  regions  arc  generally  of  light  colors,  often 
becoming  completely  white  in  winter.  This  wise  provision  of  Nature 
is  calculated  to  adapt  them  to  sustain  more  readily  the  severe  cold 
of  those  inhospitable  regions. 

Oils  and  fats  are  good  reflectors  and  bad  radiators.  Hence  we  find 
the  Laplanders  and  Esquimaux  rubbing  their  bodies  with  oils  to  pre 
vent  the  too  rapid  radiation  of  animal  heat,  whilst  the  negroes  of  the 
tropical  regions  do  the  same  thing  to  prevent  the  absorption  of  heat 
from  without. 

Snow  is  a  good  reflector  but  a  bad  absorber  and  radiator.  Hence 
it  is  that  a  layer  of  snow  in  winter  acts  to  protect  the  plants  which 
it  covers.  Snow  and  ice,  when  exposed  to  the  rays  of  the  sun.  melt 
but  slowly,  but  if  a  branch  of  a  tree  or  stone  projects  through  the 
snow,  it  causes  the  latter  to  melt  in  its  neighborhood,  first  by  absorb 
ing  the  heat  of  the  sun,  and  then  radiating  it  to  the  surrounding 
particles  of  ice  or  snow. 


Of  color?  (203)  What  is  the  object  of  clothing?  Why  are  furs  and  woollens 
suitable  to  winter?  Linens  and  cottons  to  summer  f  What  color  is  best  adapted 
to  all  seasons?  Color  of  animals  in  Arctic  regions?  Effect  of  oils  and  fats  on 
rail iiition.  and  absorption?  Examples.  Effect  of  snow?  Why  do  snow  and  ice 
melt 


CONDUCTION     OF     HEAT.  207 

If  a  stone  is  thrown  upon  a  field  of  ice,  it  soon  causes  the  ice  around 
it  to  melt,  forming  a  hole  into  which  it  sinks.  A  dark  cloth  spread 
upon  snow  acts  in  the  same  manner,  and  soon  sinks  under  the  in 
fluence  of  the  sun's  rays. 

Water  is  soonest  heated  in  a  vessel  whose  surface  is  black  and 
unpolished,  because  the  vessel  in  this  state  is  best  adapted  to  absorb 
the  heat  which  is  applied  to  it.  but  on  removing  it  from  the  fire,  the 
water  cools  rapidly.  To  retain  heat  in  liquids,  they  should  be  con 
fined  in  dense  and  polished  vessels,  as  these  are  poor  radiators. 
Hence,  for  boiling  and  cooking,  rough  and  black  vessels  should  be 
employed,  but  to  keep  the  articles  warm,  dense  and  polished  vessels 
should  be  used.  It  is  for  this  reason  that  a  silver  teapot  is  better 
than  an  earthen  one.  But  as  silver  is  a  good  conductor  of  heat,  the. 
handle  should  be  insulated  by  interposing  between  it  and  the  vessel 
some  non-conducting  substance,  as  ivory  or  bone. 

Stoves,  being  intended  to  radiate  heat,  should  be  rough  and  black, 
but  fire-places,  being  intended  to  reflect  heat  into  the  room,  should  be 
lined  with  white,  dense,  and  polished  substances,  like  glazed  earthen 
ware,  or  glazed  fire-bricks. 

Conductivity  of  Solid  Bodies. 

2O4.  CONDUCTIVITY  is  that  property  of  bodies  by  virtue 
of  which  they  transmit  heat.  Those  bodies  that  transmit 
heat  readily,  are  called  good  conductors  ;  those  that  do  not 
transmit  it  readily,  are  called  bad  conductors. 

INGENHOUSZ  showed  that  solid  bodies  possess  different 
degrees  of  conductivity,  by  means  of  an  apparatus  shown  in 
Fig.  142.  It  consists  of  an  oblong  vessel  to  contain  water, 
from  one  side  of  which  projects  a  system  of  short  tubes  for 
receiving  rods  of  different  kinds  of  solids,  such  as  metals, 
marble,  wood,  glass,  and  the  like. 

INGENHOUSZ  coated  the  different  rods  with  a  soft  wax  that 


Explain  the  effect  of  a  stone  thrown  upon  ice.  Of  a  dark  cloth  upon  snow.  Why 
i§  water  soonest  heated  in  black  and  unpolished  vessels?  In  what  vessels  is  it  best 
kept  hot?  Of  u-hat  material  should  stoves  be  constructed?  Fire-places?  Why? 
(204.)  What  is  Conductivity  ?  Good  conductors  ?  Bad  conductors  ?  Explain 
INGENHOUSZ'  apparatus. 


208 


POPULAR     PHYSICS. 


Fijr.  142 


would  melt  at  about  140°  F.,  and  then  filled  the  vessel  with 
boiling  water.  Upon  some  of  the  rods  the  wax  melted 
rapidly,  upon  some  more  slowly,  and  upon  others  not  at  all. 
This  showed  that  the  rods  varied  in  their  conductivity. 

It  has  been  shown  that  metals  are  the  best  conductors,  after 
which  comes  marble,  then  porcelain,  bricks,  wood,  glass,  resin,  &c. 

Conductivity    of    Liquids.  —  Convection. 

2O5.  Liquids  are  bad  conductors  of  heat,  except  mer 
cury,  which  is  a  metal.  They  are  such  bad  conductors  that 
RUMFORD  asserted  that  water  is  not  a  conductor  at  all. 
More  careful  experiments  have  shown  that  all  liquids  are 
conductors,  but  all  are  extremely  bad  ones. 

Liquids  are  heated  by  a  process  of  circulation  amongst 
their  particles,  called  convection,  the  heat  being  applied  from 
below,  as  shown  in  Fig.  143.  When  the  particles  at  the 
bottom  become  heated,  they  expand,  and  as  they  are  then 
lighter  than  the  cooler  particles  above  them,  they  rise  to  the 

Explain  his  method  of  using  it?  What  are,  the  best  conductors?  What  bodies 
come  next  in  order  f  (205.)  Are  liquids  good  or  bad  conductors  ?  How  arc  liquids 
heated?  Explain  the  illustration. 


CONDUCTION    OF     HEAT. 


209 


top  of  the  vessel  to  give  place  to  the  heavier  and  cooler 

ones  that  supply  their  places.     In  this  way  a  double  current 

of  particles  is  set  up,  as 

shown  in  the   figure   by 

the  arrows,  the  hot  ones 

rising  and  the  cool  ones 

descending.    This  process 

of  circulation  goes  on  till 

a  uniform  temperature  is 

imparted   to    all    of   the 

liquid. 

The  circulation  of  particles 
may  be  shown  by  putting  into 
the  vessel  particles  of  a 
substance  of  nearly  the  same 
density  as  the  liquid  :  as, 
for  example,  oak  sawdust.  Fig.  143. 

These  particles  will  partake 

of  the   motion   of  tho  fluid,  rising  up  in  the  centre,  and  descending 
along  the  walls  of  the  vessel  as  shown  in  the  figure 


Conductivity     of   Gases. 

206.  Gases  are  bad  conductors  of  heat,  but  on  account 
of  the   extreme  mobility  of  their  particles,  it  is  difficult  to 
establish  the  fact  by  direct  observation. 

Gases  are  heated  by  convection,  in  the  same  manner  as 
liquids. 

Applications   of  the   preceding   principles. 

207,  If  the   hand   be  placed   upon  different   articles  in  a  cold 
room,  they  convey  different  sensations.     Metals,  stones,  bricks,  and 
the  like,  feel  cold,  whilst  carpets,  curtains,  and  the  like,  feel  warm. 

Ifow  may  the  circulation  of  particles  le  demonstrated  ?  (  2O6  )  Arc  gases  good 
or  bad  conductors?  How  are  they  heated?  (207.)  Explain  the  different  sensa* 
tiom  experienced  on  touching  "bodies  in  a  room. 


210  POPULAR     PHYSICS. 

The  reason  of  this  is,  that  the  former  are  good  conductors,  and  readily 
abstract  the  animal  heat  from  the  hand,  whilst  the  latter  are  bad 
conductors,  and  do  not  convey  away  the  heat  of  the  hand. 

Wooden  handles  are  sometimes  fitted  to  metallic  vessels  which  arc 
to  contain  heated  liquids.  This  is  because  wood  is  a  bad  conductor, 
and  therefore  does  not  convey  the  heat,  to  the  hand.  For  a  similar 
reason,  when  we  would  handle  any  heated  body,  we  often  interpose 
a  thick  holder  of  woollen  cloth,  the  latter  being  a  bad  conductor. 

To  preserve  ice  in  summer,  we  surround  it  with  some  bad  con 
ductor,  as  straw,  sawdust,  or  a  layer  of  confined  air.  The  same 
means  are  adopted  to  preserve  plants  from  the  action  of  frost.  In 
this  case,  the  non-conducting  substance  prevents  the  radiation  of 
heat. 

Cellars  are  protected  from  frost  in  winter  by  a  double  wall 
inclosing  a  layer  of  air,  which  is  a  non-conductor.  It  is  the  layer 
of  confined  air  that  renders  double  windows  so  efficient  in  excluding 
frost  from  our  houses. 

The  feathers  of  birds  and  the  fur  of  animals  are  not  only  in  them 
selves  bad  conductors,  but  they  inclose  a  greater  or  less  quantity  of 
air,  which  renders  them  eminently  adapted  to  the  exclusion  of  cold. 

The  bark  of  trees  is  a  bad  conductor,  and  so  serves  to  protect  them 
from  the  injurious  effects  of  heat  in  summer,  and  cold  in  winter. 

Our  warmest  articles  of  clothing  are  composed  of  non-conducting 
substances,  inclosing  a  greater  or  less  quantity  of  air.  Such  are 
furs,  woollen  cloths,  and  the  like.  It  is  not  that  these  are  warm  of 
themselves,  but  they  serve  as  non-conductors,  preventing  the  escare 
of  animal  heat  from  our  bodies. 


V. LAWS      OF     EXPANSION      OF      SOLIDS,      LIQUIDS,      AND      GASES. 

Laws   of  Expansion  of  Solids. 

2O8.     Numerous  experiments  have  been  made  to  clctc"- 
mine  the  exact  amount  of  expansion  which  bodies  experience 


Why  are,  wooden  handles  attached  to  metallic,  vessels  ?  How  is  ice  preserved  in 
summer?  Why?  ffoic  are  plants  protected?  Why?  ITow  are  cellars  pro* 
tected  from  frost?  Why?  Wlnf  a^e feathers  adapted  to  exclude  cold?  Bark  of 
trees?  What  substances  form  the  wtrmest  clothing?  Why? 


LAWS     OF    EXPANSION. 


211 


by  the  addition  of  a  given  amount  of  heat.  As  in  a  former 
article,  it  will  be  found  convenient  to  consider  first,  linear 
expansion,  and  afterwards,  expansion  in  volume. 

1.  Linear  expansion.  In  order  to  compare  the  rate  of 
linear  expansion  of  different  bodies,  we  take  for  a  term  of 
comparison,  the  expansion  experienced  by  a  unit  of  length 
of  each  body  when  heated  from  32°  F.  to  33°  F.  This  is 
called  the  coefficient  of  linear  expansion. 

The  coefficients  of  linear  expansion  for  a  great  number  of 
bodies  were  determined  in  the  latter  part  of  the  last  century 
by  LAVOISIER  and  LAPLACE.  They  reduced  the  substance 
to  be  experimented  upon  to  the  form  of  a  rod  or  bar,  then 
exposed  it  fora  sufficient  time  to  the  temperature  of  melting 
ice,  and  measured  its  exact  length.  They  next  exposed  the 
bar  to  a  temperature  of  boiling  water,  and  again  measured 
its  length.  The  increased  length,  divided  by  180,  gave  the 
increase  in  length  of  the  whole  bar  for  1°  F.  This  result, 
divided  by  the  length  of  the  bar  at  32°  F.,  gave  the  linear 
expansion  of  a  unit  of  length,  and  for  an  increase  of  tem 
perature  of  1°  F.,  that  is,  the  coefficient  of  linear  expansion. 

The  following  arc  some  of  the  latest  results  : 


SUBSTANCE. 

COKFFICIENT. 

SUBSTANCE. 

COEFFICIENT. 

Glass  
Platinum  .  .  . 
Steel 

0.00000474 
0.00000-483 
O.OOOOOG31 

Brass  

Copper  .  .  . 
Silver  

0.00001044 

0.00000957 
0.00001008 

Iron  
Gold  

0.00000665 
0.00000800 

Lead  
Zinc  

0.00001565 
0.00001653 

From  the  above  table,  it  is  seen  that  the  amount  of  expansion  is 
always  very  small. 


(2O8.)  What  is  the  coefficient  of  linear  expansion  of  solids  ?    How  determined 
by  LAVOISIER  and  LAPLACE  ?    Give  some  of  the  results. 


212  POPULAR     PHYSICS. 

2.  Expansion  in  volume.  The  coefficient  of  expansion 
in  volume  is  the  increment  which  a  cubic  unit  of  the  sub 
stance  experiences  when  its  temperature  is  raised  1°  F. 
This  coefficient  may  be  determined  experimentally,  or  it 
may  be  found  by  multiplying  the  coefficient  of  linear  expan 
sion  by  3. 

Applications. 

2-09.  The  principle  of  expansion  explains  many  familiar  phe 
nomena,  some  of  which  are  indicated  below. 

A  cold  tumbler  is  often  broken  when  it  is  suddenly  filled  with 
hot  water.  The  explanation  is  simple.  Glass  is  a  bad  conductor 
of  heat,  hence  the  inside  becomes  heated  by  contact  with  the 
water  more  rapidly  than  the  outside,  and  this  inequality  of  heating 
produces  an  inequality  of  expansion  that  ruptures  the  glass.  The 
thinner  the  glass,  the  less  will  be  the  inequality  of  expansion,  and 
consequently  the  less  will  be  the  danger  of  rupture.  In  a  metallic 
vessel  such  an  accident  is  not  to  be  apprehended,  because  metals  are 
good  conductors,  and  but  little,  if  any,  inequality  of  expansion  can 
arise. 

When  a  candle  is  held  too  near  a  pane  of  glass,  the  glass  is  often 
broken ;  the  reason  is  the  same  as  before. 

Some-times  a  glass  vessel  is  broken  by  suddenly  opening  a  door 
or  window.  This  is  due  to  a  current  of  cold  air  which,  falling  upon 
the  outer  surface  of  the  glass,  causes  an  inequality  of  contraction 
that  may  produce  rupture.  All  articles  of  glass  should  be  guarded 
from  sudden  changes  of  temperature,  would  we  avoid  risk  of 
breakage. 

In  the  art  of  engineering,  it  is  important  to  take  into  account  the 
expansion  and  contractor  of  the  metals.  In  laying  the  track  of  a 
railroad,  for  example,  the  rails  should  not  be  laid  so  as  to  touch  each 
other,  otherwise  in  warm  weather  the  expansion,  acting  through  a 
long  line,  might  produce  a  force  sufficient  either  to  bend  the  rails  or 


What  is  the  coefficient  of  expansion  in  volume?  How  determined  ?  (  2O9.)  Why 
does  hot  tenter  "break  a  cold  tumbler  f  Which  is  more  entity  broken,  a  thin  rjlaM 
or  a  thick  one?  Why?  Why  is  a  pane  of  glass  broken  by  the  approach  rfa 
candle  ?  Why  may  a  nlass  retsel  ~be  broken  l>y  opening  a  floor  or  window  f  Pre 
caution*  ?  Explain  the,  effect  of  expansion  on  a  line  of  rails. 


LAWS     OF     EXPANSION.  213 

to  tear  them  from  their  fastenings.  In  employing  iron  ties  in  build 
ing,  arrangements  should  be  made  by  means  of  nuts  and  screws  to 
tighten  them  in  warm  weather,  and  loosen  them  in  cold  weather, 
otherwise  the  forces  of  contraction  and  expansion  would  weaken  and 
eventually  destroy  the  building.  Very  serious  accidents  have  oc 
curred  from  omitting  this  precaution. 

The  principle  of  expansion  and  contraction  of  metals  has  been 
utilized  in  bringing  the  walls  of  a  building  together  after  they  have 
commenced  to  separate.  A  system  of  iron  tics  is  formed,  passing 
through  the  opposite  walls,  on  the  outside  of  which  they  are  secured 
by  nuts.  The  alternate  rods  being  heated,  they  expand,  and  the 
nuts  are  screwed  up  close  to  the  walls.  On  cooling,  the  force  of 
contraction  brings  the  walls  nearer  together.  The  remaining  rods 
are  next  heated,  and  the  nuts  screwed  up.  On  cooling,  a  further 
contraction  takes  place,  and  so  on  until  the  walls  are  restored  to 
their  proper  position.  This  method  was  successfully  employed  to 
restore  the  walls  of  a  portion  of  the  Conservatoire  des  Arts  et  Metiers, 
in  Paris,  which  had  begun  to  separate. 

Compensating  Pendulum. 

21O.  The  construction  of  the  Compensating  Pendulum  depends 
upon  the  principle  of  contraction  and  expansion  of  metals.  We  have 
?cen  already  that  the  time  of  oscillation  of  a  pendulum  depends  upon 
its  length,  vibrating  faster  when  shortened,  and  slower  when  length 
ened.  Inconsequence  of  variations  of  temperature,  if  a  pendulum 
were  suspended  by  a  single  metallic  rod,  its  rate  of  vibration  would 
be  continually  changing. 

To  obviate  this  defect  and  secure  uniformity  of  rate,  various  de 
vices  have  been  employed,  one  of  the  most  important  of  which  is 
HARRISON'S  Gridiron  Pendulum,  shown  in  Fig.  144.  It  consists  of 
five  parallel  bars  of  rnetal.  arranged  as  shown  in  the  figure.  The 
bars  a,  b.  c,  and  d,  are  of  steel,  and  when  they  expand,  the  effect  is 
to  lengthen  the  pendulum  ;  the  bar.  d.  passes  freely  through  the 
cross  piece,  or.  and  is  firmly  attached  to  the  piece,  mn.  The  bars, 


Precautions  to  ~be  taken  in  "building  with  iron  ?  Explain  the  method  of  straight 
ening  walls.  (2O1.)  What  effect  has  heat  upon  a  pendulum?  IIww  are  its 
defects  remedied?  Explain  the  theory  anti  construction  of  HARRISON'S  Gridiron 
Pendulum. 


214 


POPULAR     PHYSICS. 


h  and  £,  are  of  brass,  firmly  attached  to  both 
of  the  cross  pieces,  mn  and  or.  When  they 
expand,  the  effect  is  to  raise  the  piece,  mn: 
and  thus  to  shorten  the  pendulum. 

If  the  pieces  are  properly  adjusted,  the 
amount  of  shortening  is  exactly  equal  to  he 
amount  of  lengthening  before  mentioned,  and 
these  two  balancing  each  other,  the  length  of 
the  pendulum  remains  invariable.  The  ad 
justment  requires  that  the  lengths  of  the  rods 
should  be  inversely  as  their  coefficients  of  linear 
expansion. 

Laws  of  Expansion   of  Liquids. 

211.  Liquids  are  much  more  expansi 
ble  than  solids,  on  account  of  their  feeble 
cohesion ;  their  expansion  is  also  much 
more  irregular,  especially  when  their  tem 
perature  approaches  the  boiling  point. 

The  expansion  of  a  liquid  may  be  ab 
solute  or  relative.  The  absolute  expan 
sion  of  a  liquid  is  its  actual  increase  of 
volume;  the  relative  expansion  is  its 
increase  of  volume  with  respect  to  the 
containing  vessel.  For  example,  in  a 
thermometer  the  rise  of  the  liquid  in  the  stem  is  due  to  its 
relative  expansion,  with  respect  to  that  of  the  stem.  Both 
expand,  but  the  liquid  more  rapidly  than  the  glass.  The 
capacity  of  the  bulb  increases  with  an  increase  of  heat,  but 
the  volume  of  its  contained  mercury  increases  more  rapidly, 
and  therefore  rises  in  the  stem.  The  absolute  is  usually 
greater  than  the  relative  expansion.  It  is  the  relative  ex 
pansion  that  we  generally  observe. 


Fig.  144. 


(211.)  Why  are  liquids  more  expansible  than  solids?    What  is  absolute  expan 
sion?     Relative   expansion?    Example.     Which  is   generally  observed? 


LAWS     OF    EXPANSION.  215 

The  coefficient  of  expansion  of  a  liquid  is  the  expansion 
of  a  unit  of  volume,  corresponding  to  an  increase  of  temper 
ature  of  one  degree. 

Taken  with  reference  to  glass,  the  coefficient  of  expansion 
for  mercury  is  0.000833  ;  that  of  water  is  three  times  as 
great,  and  that  of  alcohol  nearly  eight  times  as  great  as  that 
<of  mercury. 

Maximum   Density   of  Water. 

212.  If  water  is  cooled  down  gradually,  its  volume  con 
tinues  to  contract  until  it  reaches  the  temperature  of 
39.°2  F.,  when  it  attains  its  maximum  density.  If  it  be 
still  further  cooled  it  begins  to  expand,  and  at  32°  F.  it  be 
comes  solid,  or  freezes. 

This  curious  phenomenon  may  be  shown  by' using  a  water 
thermometer  in  connection  with  a  mercurial  one.  As  the 
temperature  is  diminished,  the  liquids  descend  in  the  stems 
of  both  thermometers  until  the  mercurial  one  shows 
39.°2  F.,  after  which,  if  the  cooling  process  be  continued, 
the  mercury  will  continue  to  fall,  whilst  the  water  will  begin 
to  rise. 

This  apparent  exception  to  the  law  of  expansion  and  con 
traction  is  explained  from  the  fact,  that  at  the  temperature 
of  39.°2  F.,  the  particles  begin  to  arrange  themselves  in  a 
new  order,  preparatory  to  taking  a  crystalline  form.  Some 
other  substances,  such  as  melted  iron,  sulphur,  bismuth,  &c., 
exhibit  a  similar  expansion  of  volume  immediately  previous 
to  taking  a  solid  crystalline  form.  It  is  this  property  of 
expanding  at  the  time  of  crystallization,  that  renders  iron 
so  valuable  a  metal  for  casting.  The  expansion  of  the  metal 
acts  to  fill  the  mould,  thus  giving  sharpness  and  accuracy  to 
the  casting. 


What  is  the  coefficient  of  expansion  of  a  liquid  ?  "What  is  its  value  for  mercury 
with  reference  to  glass?  How  do  the  coefficients  of  water  and  alcohol  compare  with 
mercury?  (212)  At  what  temperature  has  water  the  greatest  density?  When 
does  it  freeze?  How  may  the  phenomenon  be  shown?  How  explained?  "What 
other  bodies  exhibit  similar  phenomena?  "Why  is  iron  so  valuable  for  casting? 


210  POPULAR    TIIYSICS. 

The  fact  that  water  has  its  greatest  density  at  39°  2  F.,  causes 
ice  to  form  at  the  surface  instead  of  at  the  bottom  of  rivers  and  lakes. 
Were  it  not  that  ice  is  lighter  than  water,  it  would  sink  to  the 
bottom  as  fast  as  formed,  or  rather  would  form  at  the  bottom,  and 
in  the  colder  regions  of  the  globe  would  soon  convert  entire  lakes 
into  solid  masses  of  ice.  As  ice  and  water  are  bad  conductors  of 
heat,  the  summer  sun  would  not  posesss  the  power  to  convert  them 
again  into  \vater. 

In  Switzerland  it  is  found  by  experiment  that  the  temperature  of 
the  water  at  the  "bottom  of  deep  and  snow-fed  lakes  remains  during 
the  entire  year  at  the  uniform  temperature  of  39°.2  F.,  although 
the  surface  is  frozen  in  winter,  and  in  summer  rises  to  75°  or  80°  F. 

It  is  because  water  has  its  maximum  density  at  39°. 2  F.,  that  it 
is  taken  at  this  temperature,  as  the  standard  of  comparison  for  deter 
mining  the  specific  gravity  of  bodies. 

Law   of  expansion  of  Gases. 

213.  Gases  arc  not  only  more    expansible  than  solids 
and  liquids,  but  they  also  expand  more  uniformly. 

The  coefficient  of  expansion  of  a  gas,  is  the  expansion 
which  a  unit  of  volume  experiences  when  its  temperature  is 
increased  one  degree. 

GAY  LUSSAC  supposed  that  all  gases  expand  equally  for 
equal  increments  of  temperature ;  but  more  recent  investi 
gations  show  that  the  coefficients  of  expansion  are  slightly 
different  for  different  gases.  This  difference  is,  however,  so 
small,  that  for  all  practical  purposes  we  may  regard  all  gases 
as  having  the  same  coefficient.  The  value  of  the  coefficient 
of  expansion  for  gases  is  0.00204,  which  is  about  eight  times 
that  of  water. 

Applications. 

214.  The  law  of  expansion  of  gases  when  heated,  has  many 
important  applications,  some  of  which  will  be  explained. 

Explain  the  consequences  of  the  expansion  of  water  on  freezing.  Example  of 
Ic  lake*  in  Switzerland.  Why  in  icater  taken  at  ;>i'c.  "2  F.  <f*  a  (standard? 
213-)  What  bodies  are  most  expansible?  What  is  the  coelHciont  of  expansion? 
was  GAY  LUSSAC'S  opinion?  Was  it  strictly  correct? 


LAWS     OF    EXPANSION.  217 

When  the  air  of  a  room  becomes  warmed  and  vitiated  by  the 
presence  of  a  number  of  persons,  it  expands  and  becomes  Lighter  than 
the  external  air:  hence  it  rises  to  the  top  of  the  room,  and  its  place 
is  supplied  by  fresh  air  from  without,  which  enters  through  the 
cracks  of  the  doors,  or  through  apertures  constructed  for  the  purpose. 
Openings  should  be  made  at  the  upper  part  of  the  room  to  permit 
the  foul  air  to  escape.  Such  is  the  theory  of  ventilation  of  rooms. 

In  large  buildings,  like  theatres,  the  spectators  in  the  upper 
galleries  often  experience  great  inconvenience  from  the  hot  and 
corrupt  air  arising  from  below.  To  remedy  this  evil,  large  open 
ings,  called  ventilators,  should  be  constructed  in  the  ceiling,  and  cor 
responding  openings  should  be  arranged  near  the  bottom  of  the  build 
ing,  to  supply  a  sufficient  quantity  of  fresh  air  to  keep  up  the  circu 
lation. 

The  principal  of  expansion  gives  a  draft  to  our  chimneys.  The 
hot  air  ascends  through  the  flue,  and  its  place  is  supplied  by  a  con 
tinued  current  of  cold  air  from  below,  which  keeps  up  the  com 
bustion  in  the  fire-place  or  grate. 

The  same  principle  is  applied  in  warming  buildings  by  means  of 
furnaces.  Furnaces  are  placed  in  the  lowest  story  of  the  building, 
and  are  provided  with  air  chambers,  which  communicate  with  the 
external  air  by  means  of  air-pipes.  When  the  air  becomes  heated 
in  the  air  chamber,  it  rises  through  pipes,  or  flues  in  the  walls,  to 
the  upper  stories  of  the  building,  and  is  admitted  to  or  excluded  from 
the  different  apartments  by  valves,  called  registers. 

The  principle  of  expansion  of  air  explains  many  meteorological 
phenomena.  When  the  air  in  any  locality  becomes  heated  by  the 
rays  of  the  sun,  it  rises  and  its  place  is  supplied  by  colder  air  from 
the  neighboring  regions,  thus  producing  the  phenomena  of  winds. 
The  circulation  of  the  atmosphere  in  the  form  of  winds,  tends  to 
equalize  the  temperature,  and  also,  by  transporting  clouds  and 
vapors,  tends  to  equalize  the  distribution  of  water  over  the  globe. 

Winds  also  serve  to  remove  the  vitiated  air  of  cities,  replacing  it 
by  the  pure  air  of  the  neighboring  places,  thus  contributing  to  the 
preservation  of  life  and  health.  Winds  also  act  to  propel  vessels  on 

(  214.)  How  does  the  principle  of  expansion  operate  in  ventilation  ?  How  are 
large  buildings  ventilated?  What  gives  draft  to  chimneys?  Explain  the  theory 
of  heating  ~by  furnaces.  How  does  the  principle  of  expansion  produce  winds  f 
Their  effect  on  distribution  of  warmth  and  moisture  f 

JO 


218  POPULAR     PHYSICS. 

the  ocean,  thus  contributing  to  the  spread  of  commerce  and  civiliza 
tion. 

Without  wind?,  our  cities  would  become  centres  of  infection,  the 
clouds  would  remain  motionless  over  the  localities  where  they  were 
formed,  the  greater  portion  of  the  earth  would  become  arid  and  desert, 
without  rivers  or  streams  to  water  them,  and  the  whole  earth  would 
s  on  become  uninhabitable. 


Density  of  Gases. 

215.  The  density  of  a  gas  depends  upon  the  pressure  to 
which  it  is  subjected,  and  also  upon  its  temperature. 

It  is  for  this  reason  that  we  select  as  a  term  of  comparison 
the  density  at  some  particular  pressure  and  temperature. 
The  standard  pressure  is  that  of  the  atmosphere  when  the 
barometer  stands  at  30  inches,  and  the  standard  temperature 
is  32°  F.,  or  the  freezing  point  of  water.  To  determine 
the  density  at  any  other  pressure,  we  apply  MARIOTTE'S 
law  ;  to  determine  it  at  any  other  temperature,  we  apply 
the  coefficient  of  expansion,  as  explained  in  preceding 
articles. 

Suppose  it  were  required  to  determine  the  density  of  air  when  the 
barometer  indicates  20  inches,  and  the  thermometer  62°  F.,  Ihe 
density  being  equal  to  1  at  the  standard  temperature  and  pressure. 
The  pressure  being  only  two  thirds  the  standard  pressure,  the  air  in 
the  case  considered  would  occupy  once  and  a  half  its  primitive 
volume,  supposing  the  temperature  to  remain  at  32°  F.  But  the 
temperature  being  62°  F.,  or  30°  above  the  standard,  we  multiply 
1.5  by  30  times  0.00204  for  the  expansion.  This  product,  added  to 
1.5,  gives  for  a  result,  1.5918.  That  is,  a  unit  of  volume  at  the 
standard  pressure  and  temperature  becomes  1.5918  units  of  volume 
at  the  given  pressure  and  temperature.  Because  the  density  va 
ries  inversely  as  the  volume,  we  shall  have  for  the  required  density, 

or  °-6282- 


Other  effects  of  winds?  (215.)  On  what  does  the  density  of  a  gas  depend? 
What  do  we  take  as  a  standard  ?  How  do  we  determine  the  density  at  any  other 
pressure  and  temperature  ?  Example. 


MELTING    AXD    FREEZING. 


219 


The  following  table  exhibits  the  density  of  some  of  the  most  im 
portant  gases,  air  being  taken  as  a  standard  : 


TABLE. 


GAS. 

DENSITY. 

GAS. 

DENSITY. 

Air 

1  0000 

Oxygon 

1.1056 

Hydrogen  

0.0692 

Carbonic  acid 

1.5290 

Nitrogen  

0.9714 

Hydrogen  is  the  lightest  known  body,  its  density  being  fourteen 
and  a  half  times  less  than  that  of  air. 


VI.— CHANGE   OF   STATE   OF  BODIES  BY  THE  ACTION  OF  HEAT. 


Fusion. 

216.  It  has  been  stated  that  heat  not  only  causes  bodies 
to  expand,  but  that  it  may  in  certain  circumstances  cause 
them  to  change  from  the  solid  to  the  liquid  state,  or  from 
the  liquid  to  the  gaseous  state. 

When  a  body  passes  from  a  solid  to  a  liquid  state,  it  is 
said  to  melt,  or  fuse,  and  the  act  of  changing  state  in  this 
case  is  espied,  fusion. 

If  a  melted  body  is  suffered  to  cool,  it  becomes  solid  at 
the  same  temperature  at  which  it  melted.  Hence  the  melt 
ing  point  is  the  same  as  the  freezing  point. 

Fusion  takes  place  when  the  force  of  cohesion,  which  "holds  the 
particles  of  a  body  together,  is  exactly  balanced  by  the  heat  which 
tends  to  separate  them.  The  temperature  at  which  fusion  takes 
place  is  different  for  different  bodies.  For  some  bodies  it  is  very  low, 
and  for  others  very  high,  as  is  shown  in  the  following 

What  is  the  lightest  body?  Give  the  densities  of  some  other  gases.  (216.) 
What  is  melting  or  fusion  ?  When  does  fusion  take  place  ?  Is  the  melting  point 
the  same  for  all  solids  f 


220 


POPULAR     PHYSICS. 
TABLE. 


BODY. 

TEMPERATURE 
OF  FUSION. 

BODY. 

TEMPERATURE 
OF  FUSION. 

Mercury 

—  39°  F. 

Bismuth 

500°    F, 

Ice. 

32° 

Lead 

627° 

Tallow 

91° 

Antimony 

842° 

White  wax  

149° 

Zinc 

932° 

Sulphur..    . 

232° 

Silver 

1832° 

Tin 

455° 

Gold 

2282° 

All  bodies  are  not  melted  by  the  action  of  heat.  Some  are  de 
composed,  such  as  paper,  wood,  bone,  marble,  &c.  Simple  bodies, 
that  is,  bodies  which  are  composed  of  but  one  kind  of  matter,  always 
melt,  if  sufficiently  heated,  with  a  single  exception.  Carbon  has  thus 
far  resisted  all  attempts  to  fuse  it. 

Latent   Heat  of  Fusion. 

2 IT.  Bodies  which  can  be  melted  always  present  the 
remarkable  phenomenon,  that  when  they  are  heated  to  the 
temperature  of  fusion,  they  can  not  be  heated  any  higher 
until  the  fusion  is  complete.  For  example,  if  ice  be  exposed 
to  heat,  it  begins  to  melt  at  32°  F.,  and  if  more  heat  be 
applied,  the  melting  is  accelerated,  but  the  temperature  of 
the  mixture  of  ice  and  water  remains  at  32°  until  all  the  ice 
is  melted. 

The  heat  that  is  applied  during  the  process  of  fusion, 
enters  into  the  body  without  raising  its  temperature,  and  is 
said  to  become  latent.  When  the  body  returns  to  its  solid 
state,  all  the  latent  heat  is  again  given  out,  and  once  more 
becomes  sensible. 

The  phenomenon  of  latent  heat  may  be  illustrated  by  the  follow 
ing  experiment.  If  a  pound  of  pulverized  ice.  at  32°  F..  be  mixed 

Examples.  Are  all  bodies  melted  ~by  the  action  of  heat  t  Examples.  (21 7-) 
What  is  latent  heat  ?  Sensible  heat  ?  How  may  the  phenomenon  of  latent  heat  l>6 
illustrated. 


MELTING    AND    FKEEZING.  221 

with  a  pound  of  water  at  174°  F.,  the  heat  of  the  water  will  be  just 
sufficient  to  melt  the  ice,  and  there  will  result  two  pounds  of  water 
at  the  temperature  of  32°  F.  During  the  process  of  melting,  142°  of 
heat  have  been  absorbed  and  become  latent  •  hence,  we  say  that  the 
heat  required  to  melt  ice  at  32°  F.  is  142°,  01-5  in  other  words, 
the  latent  heat  of  water  at  32°  is  142°. 

The  enormous  amount  of  heat  which  becomes  latent  when  ice 
melts,  explains  why  it  is  that  large  masses  of  ice  remain  unmeltcd 
for  a  considerable  time  after  the  temperature  of  the  air  is  raised 
above  32°  F.  Conversely,  the  immense  quantity  of  heat  evolved 
when  water  passes  to  the  state  of  ice,  explains  why  it  is  that  ice 
forms  so  slowly  in  extremely  cold  weather.  The  absorption  of  heat 
in  melting,  and  production  of  heat  in  freezing,  tend  to  equalize  the 
temperature  of  climates  in  the  neighborhood  of  large  masses  of 
water,  like  lakes  and  rivers. 

Congelation. — Solidification. — Regelation. 

218.  Any  body  that  can  be  melted  by  the  application 
of  heat,  can  be  brought  back  to  a  solid  state  by  the  abstrac 
tion  of  heat.  This  passage  from  a  liquid  to  a  solid  state  is 
called  congelation,  or  solidification. 

In  every  body,  the  temperature  at  which  congelation  com 
mences,  is  the  same  as  that  at  which  fusion  begins.  Thus,  if 
water  be  cooled,  it  will  begin  to  congeal  at  32°  F.,  and  con 
versely,  it  ice  be  heated,  it  will  begin  to  melt  at  32°  F. 
Furthermore,  the  amount  of  heat  given  out,  or  rendered 
sensible  in  congealing,  is  exactly  equal  to  that  absorbed,  or 
rendered  latent  in  melting. 

Some  liquids  can  not  be  congealed  by  the  greatest  cold  to  which 
we  can  subject  them  ;  such  are  alcohol  and  ether.  Pure  water  con 
geals  at  32° :'  the  salt  water  of  the  ocean  congeals  at  27°  :  olive  oil 
at  21°  -.  linseed  and  nut  oils  at  17°. 


Explain  the  action  of  latent  heat  on  melting  masses  of  ice.  Also  on  freezing 
masses  of  water.  (218)  What  is  congelation  ?  Haw  does  the  point  of  congelation 
compare  with  that  of  fusion  ?  Illustrate.  How  does  the  heat  given  out  in  solidifying 
compare  with  that  taken  up  in  melting?  What  liquids  have  never  been  frozen  ? 


POPULAR    PHYSICS. 

Water  reaches  its  maximum  densitj^  at  38°.  75,  and  as  its  temper 
ature  is  diminished  from  this  limit,  its  volume  continues  to  increase 
until  congelation  is  completed. 

If  two  smooth  pieces  of  melting  ice  be  pressed  against 
each  other  they  are  soon  frozen  together.  This  phenome 
non  is  called  regelation. 

Regelation  is  explained  by  supposing  the  interior  of  the  ice  colder 
than  the  outer  layer  just  passing  into  the  state  of  water.  When  the 
pieces  are  pressed  together  the  layer  of  water  at  32°  F.  has  a  colder 
body  on  each  side.  The  latent  heat  of  fusion  of  this  layer  is  soon 
absorbetl  and  conducted  away,  and  the  water  is  converted  into  ice. 
The  formation  of  a  snow-ball  depends  on  regelation.  Below  a  tem 
perature  of  32°  F.  the  particles  of  snow  are  dry  and  regelation  can 
not  take  place.  Hence  a  coherent  snow-ball  can  only  be  made  of 
melting  snow. 

Crystallization. 

219.  When  bodies  pass  slowly  from  the  liquid  to  the 
solid  states,  their  particles,  instead  of  arranging  themselves 
in  a  confused  manner,  tend  to  group  themselves  into  regular 
forms.  These  forms  are  called  crystals,  and  the  process  of 
forming  them  is  called  crystallization. 

Flakes  of  snow,  sugar  candy,  alum,  common  salt,  and  the 
like,  offer  examples  of  crystallized  bodies.  The  forms  of  the 
crystals  are  best  seen  under  a  magnifying  glass. 

Bodies  may  be  crystallized  in  two  different  ways.  In  the 
first  case,  AVC  melt  them,  and  then  allow  them  to  cool  slowly. 
If  a  vessel  of  sulphur  be  melted  and  allowed  to  cool  slowly, 
it  will  commence  crystallizing  about  the  surface,  and  if  we 
break  the  crust  thus  formed,  and  pour  out  the  interior  liquid 
sulphur,  we  may  obtain  beautiful  crystals  of  sulphur. 

In  the  second  case,  we  dissolve  the  body  to  be  crystallized 
and  then  allow  the  solution  to  evaporate  slowly.  The  dis 
solved  body  is  then  deposited  at  the  bottom  and  on  the 

Explain  the  phenomenon  of  regelation.  Illustrate.  (219.)  What  are  crystals  ': 
What  is  crystallization  ?  Examples.  How  many  methods  of  crystallization  ? 
Explain  the  first  method.  The  second  method. 


VAPORIZATION.  223 

sides  of  the  vessel  in  the  form  of  crystals.  The  slower  the 
process,  the  finer  will  be  the  crystals.  It  is  in  this  manner 
that  we  crystallize  candy  and  various  salts. 

Freezing  Mixtures. 

22O.  The  absorption  of  heat  which  takes  place  when  a 
body  passes  from  a  solid  to  a  liquid  state,  is  often  utilized  in 
the  production  of  intense  cold.  This  result  is  best  obtained 
by  mixing  certain  substances,  and  these  mixtures  are  then 
called  freezing  mixtures. 

A  mixture  of  one  part  of  common  salt  and  two  parts  of 
pounded  ice  forms  a  mixture  that  is  used  for  freezing  cream. 
The  salt  and  ice  have  an  affinity  for  each  other,  but  they 
can  not  unite  until  they  pass  to  the  liquid  state  ;  in  order  to 
pass  to  this  state  they  absorb  a  great  quantity  of  heat  from 
the  neighboring  bodies,  and  this  causes  the  latter  to  freeze. 
By  means  of  a  mixture  of  salt  and  snow,  the  thermometer 
may  be  reduced  to  0. 


VII. — Y  A  P  O  R  I  Z  AT  I  O  N.  —  ELASTIC      FORCE      OF      VAPORS. 

Vaporization,  —  Volatile   and  Fixed  Liquids. 

221.  When  sufficient  heat  is  applied  to  a  liquid,  it  is 
converted  into  a.  gaseous  form  and  is  called  a  vapor.  The 
change  of  state  from  a  liquid  to  a  gaseous  state  is  called 
vaporization. 

Conversely,  if  heat  be  abstracted  from  a  vapor,  it  will 
return  to  a  liquid  form.  The  change  of  state  from  a  vapor 
ous  to  a  liquid  form  is  called  condensation. 

Vapors  are  generally  colorless,  and  are  endowed  with  an 
expansive  force,  or  tension,  which,  when  heated,  may  become 
very  great. 

(22O.)  What  is  a  freezing  mixture?  Example.  Explain  its  action?  (221.) 
What  is  vaporization  ?  Condensation  ?  General  properties  of  vapors  ? 


22  ±  POPULAR     PHYSICS. 

The  number  of  vapors  that  exist  at  ordinary  temperatures 
is  very  small.  Of  these,  watery  vapor  is  the  most  familiar, 
as  well  as  the  most  important,  on  account  of  the  part  which 
it  plays  in  many  natural  phenomena. 

Liquids  are  divided  into  two  classes,  with  respect  to  the 
readiness  with  which  they  pass  from  the  liquid  to  the  vapor 
ous  state,  viz. :  volatile  liquids  and  fixed  liquids. 

VOLATILE  LIQUIDS  are  those  which  have  a  natural  tendency 
to  pass  into  a  state  of  vapor  even  at  ordinary  temperatures, 
such  as  ether,  alcohol,  and  the  like.  If  a  vessel  of  water, 
alcohol,  ether,  or  chloroform  be  left  exposed  to  the  air,  the 
liquid  is  slowly  converted  into  vapor,  and  disappears;  in 
other  words,  it  evaporates.  To  the  class  of  volatile  liquids 
belong  essences,  essential  oils,  volatile  oils,  amongst  which 
may  be  mentioned  spirits  of  turpentine,  oil  of  lavender, 
attar  of  roses,  oil  of  orange,  and  the  like. 

FIXED  LIQUIDS  are  those  which  do  not  pass  into  vapor  at 
any  temperature,  as,  for  example,  fish  oils,  olive  oils,  and  the 
like.  At  high  temperatures  they  are  decomposed,  giving 
rise  to  various  kinds  of  gases,  but  to  no  true  vapors  that  can 
be  condensed  into  the  original  form  of  the  liquid.  Some 
oils,  like  linseed  oil,  harden  on  exposure  to  the  air,  but  it  is 
not  by  evaporation,  but  by  absorbing  oxygen  from  the  air, 
and  thus  passing  to  a  solid  state.  Some  solids  are  capable 
of  passing  directly  to  a  state  of  vapor  without  first  becoming 
liquid.  To  this  class  belong  camphor,  musk,  and  odorous 
bodies  generally.  Snow  and  ice  may,  under  certain  circum 
stances,  evaporate  without  melting. 

Evaporation  under  pressure. 
222.     The  influence  of  evaporation  by  pressure  may  be  illustrated 


The  most  important  vapor  ?  What  t\vo  classes  of  liquids  have  we  ?  What  aro 
volatile  liquids?  Examples.  Illustrate.  What  are  fixed  liquids?  Examples. 
Effect  of  high  temperatures  upon  them  ?  Give  examples  of  solids  that  vaporize  ? 


VAPORIZATION. 


225 


by  means  of  an  apparatus  shown  in  Fig.  145.  It  consists  of  a  curved 
tube,  the  short  branch  of  which  is  closed  and  filled  with  mercury  ; 
the  mercury  also  fills  a  portion  of  the  long  branch.  A  small  quantity 
of  ether  is  introduced  into  the  short  branch,  when  it  at  once  rises  to 
the  top.  5,  of  this  branch.  At  ordinary  temperatures,  the  pressure 
of  the  external  atmosphere  exerted  through  the  mercury,  is  sufficient 
to  prevent  the  ether  from  forming  vapor 

If.  however,  the  tube  is  plunged 
into  a  vessel  of  water  heated  to 
112°,  the  ether  will  be  converted 
into  vapor  and  will  occupy  a  cer 
tain  portion,  AB^  of  the  tube, 
holding  in  equilibrium  the  pressure 
of  the  atmosphere,  together  with 
the  weight  of  the  mercurial  column 
whose  height  is  AC. 

If  the  tube  be  withdrawn  and 
allowed  to  cool,  the  vapor  of  ether 
will  be  condensed,  and  will  appear 
as  a  liquid  at  B.  If  more  heat  be 
applied,  it  will  again  be  converted 
into  vapor,  and  the  mercury  will 
rise  in  the  branch,  C,  as  long  as 
any  ether  remains  to  be  evaporated. 
This  shows  that  the  tension  of  the 
vapor  augments  with  the  tempera 
ture.  This  principle  holds  true  for 
all  kinds  of  vapor. 

The  tension  acquired  by  the  va 
por  of  water,  or  steam,  often  be 
comes  so  great  by  being  heated  as 
to  burst  the  strongest  vessels,  and 

thus  is  the  cause  of  frightful  accidents.  The  cause  of  wood  snapping 
when  burned  in  a  fire-place,  is  the  expansion  of  the  water  in  the 
pores,  giving  rise  at  last  to  an  explosion.  When  a  chestnut  is 
roasted  in  the  ashes,  the  moisture  within  the  shell  expands  into 

(222.)  Explain  the  experiment  showing  the  influence  of  pressure  on  vaporiza 
tion.  Why  does  iwod  map  when  burned?  Why  does  a  chestnut  snap  when 
roasted  t 


Fig.  145. 


POPULAR     PHYSICS. 


steam,  and  explodes  with  sufficient  force  to  throw  the  nut  from  the 
fire.  Hence  it  is  that  a  small  puncture  is  usually  made  in  the  shell, 
which  permits  the  escape  of  the  steam  and  prevents  explosion 

Instantaneous   Evaporation  in   a  Vacuum. 

223.  Vapors  formed  upon  the  surface  of  a  liquid  escape 
by  virtue  of  their  tension.  Under  ordinary  circumstances, 
the  pressure  of  the 
air  prevents  a  very 
rapid  escape  of  va 
por  at  ordinary 
temperatures,  but 
when  the  atmos 
pheric  pressure  is 
diminished  in  any 
way,  evaporation 
takes  place  with 
great  rapidity.  If 
the  pressure  is  en 
tirely  removed,  the 
evaporation  is  in 
stantaneous,  like 
the  flash  of  gun 
powder,  especially 
\f  the  liquid  is  very 
volatile. 

This  principle  may 
be  illustrated  by 
means  of  the  appara- 
I  us  shown  in  Fig.  1-46. 

ft  consists  of  several  barometer  tubes,  J,  7?,  C,  D,  filled  with  mercury, 
and  inverted  in  a  common  cistern  of  meicury.  as  shown  in  the 
figure.  The  whole  apparatus  is  supported  by  a  frame,  to  which  is 

How  remedied?  (223.)  "Why  do  vapors  escape  from  the  surfaces  of  liquids! 
When  the  pressure  is  removed,  what  happens  ?  flow  may  the  principle  ~be  illus 
trated?  Explain  the  experiment  in  detail. 


VAPORIZATION. 


227 


attached  a  graduated  scale.  The  mercury  will  stand  at  the  same 
height  in  all  of  the  tubes,  at  the  height  in  J,  for  example. 

If  a  few  drops  of  water  be  introduced  into  the  tube  5,  they  will 
rise  through  the  mercury  in  the  tube,  and  on  reaching  the  vacuum, 
will  be  instantly  converted  into  vapor,  as  is  shown  by  the  depression 
that  takes  place  in  the  column  of  mercury.  If  a  little  alcohol  be 
introduced  into  the  tube  C,  it  will,  in  like  manner,  be  converted 
into  vapor,  and  will  produce  a  still  greater  depression  of  the  column. 
If  a  small  quantity  of  ether  be  introduced  into  the  tube  I),  a  still 
greater  depression  of  the  mercury  will  be  observed. 

This  experiment  shows  that  the  tension  of  the  vapor  of  ether  is 
greater  than  that  of  alcohol,  and  that  of  alcohol  greater  than  that  of 
water.  By  careful  measurement,  it  is  found  that  the  tension  of  the 
vapor  of  ether  is  twenty-five  times  as  great  as  that  of  water,  and 
six  times  as  great  as  that  of  alcohol. 

Limit   of  the  Tension   of  Vapors. 

224.  If  a  sufficient  quantity  of  each  of  the  liquids  in  the 
last  experiment   be  introduced  into  the  tubes,  vapor  will 
finally  cease  to  form,  and  a  portion  will  remain  in  the  liquid 
state.     In  this  case  the  tension  of  the  vapor  already  formed 
is  sufficient  to  balance  the  tendency  of  the  liquid  to  pass  into 
a  state  of  vapor.     In  this  state  of  affairs  no  more  vapor  can 
form  without  a  change  of  temperature.     This  is  the  case 
supposed  in  the  last  article. 

Saturation. 

225.  When  a  given  space  has  taken  all  of  the  vapor 
that  it  can  contain,  it  is  said  to  be  saturated.     For  example, 
if  water  be  poured  into  a  bottle  filled  with  dry  air,  and  the 
bottle  be  hermetically  sealed,  a  slow  evaporation  will  go  on 
until  the  tension   of  the  vapor  given  off  is  equal  to  the 
tendency  of  the  remaining  water  to  pass  into  vapor,  when  it 

What  does  the  experiment  show?     (224.^  When  does  vapor  cease  to  form? 
225.)  When  is  a  space  saturated  with  vapor?    Example  ? 


228  POPULAR   PHYSICS. 

will  cease.  In  this  case,  the  space  within  the  bottle  is 
saturated. 

It  is  a  remarkable  fact,  established  by  numerous  experi 
ments,  that  for  the  same  temperature,  the  quantity  of  watery 
vapor  necessary  to  saturate  a  given  space  is  always  the 
same,  whether  th^it  space  is  a  vacuum,  or  whether  it  contain 
air  or  any  other  gas.  The  only  point  of  difference  in  these 
cases  is  the  rapidity  with  which  the  saturation  takes  place. 

If  the  temperature  varies,  the  amount  of  vapor  required 
to  saturate  a  given  space  will  vary  also.  The  higher  the 
temperature,  the  greater  will  be  the  quantity  of  vapor 
required  to  saturate  the  given  space,  and  the  lower  the 
temperature,  the  less  the  quantity  required  for  saturation. 

The  quantity  of  watery  vapor  in  the  atmosphere  is  very 
variable,  but  notwithstanding  the  continued  evaporation 
that  is  taking  place  from  lakes,  rivers,  and  oceans,  the  air  in 
the  lower  regions  of  the  atmosphere  is  never  saturated. 
The  reason  is,  that  the  vapor  being  less  dense  than  the  air 
at  the  surface,  rises  into  the  higher  regions,  where  it  is  con 
densed  by  the  greater  cold  existing  there,  and  falls  to  the 
earth  in  the  form  of  rain. 

Causes  that  accelerate  Evaporation. 

226.  The  slow  evaporation  of  water  on  the  surface  of 
our  globe  is  accelerated  by  many  causes,  some  of  which  are 
indicated  below : 

1.  Temperature. — Increase  of  temperature  also  increases 
the  tension  of  the  vapor  formed,  and  accelerates  evapora 
tion. 

This  property  is  utilized  in  the  arts  in  the  manufacture  of  extracts. 
The  evaporation  is  carried  on  in  chambers  kept  at  temperatures  of 

What  is  the  law  of  saturation  at  a  given  temperature  ?  What  effect  has  a  change 
of  temperature  ?  Why  is  the  amount  of  vapor  in  the  atmosphere  variable?  (  226.) 
What  effect  has  increase  of  temperature  on  evaporation  ?  How  is  this  property 
utilized  ? 


VAPORIZATION.  229 

from  80°  to  140°  F.,  the  air  being  continually  renewed  to  carry  off 
the  vapor  as  fast  as  formed. 

2.  Pressure. — Diminution  of  pressure  facilitates  evapora 
tion. 

This  principle  has  been  utilized  in  the  arts  for  the  concentration 
Df  syrups.  This  application  is  illustrated  by  the  method  of  concen 
trating  syrups  in  sugar  refining.  The  syrups  are  placed  in  large 
spherical  boilers,  from  which  the  air  is  extracted  by  means  of  air- 
purnps  worked  by  steam. 

3.  Change  of  air. — A  continual  change  of  the  air  in  con 
tact  with  the  liquid  facilitates  evaporation,  by  carrying  off 
the  vapor  which  would  otherwise  saturate  the  layer  in  con 
tact  with  the  liquid,  and  effectually  check  the  formation  of 
additional  vapor. 

It  is  for  this  reason  that  the  surface  moisture  of  our  fields  and 
roads  disappears  more  rapidly  when  there  is  a  breeze  than  in  calm 
weather.  In  the  arts,  the  principle  is  applied  by  keeping  a  current 
of  air  playing  across  the  surface  of  the  liquid  to  be  evaporated,  by 
means  of  blowers,  or  otherwise. 

4.  Extent  of  the  liquid. — A  large  surface  is  favorable  to 
rapid  evaporation,  by  affording   a   great  number  of  points 
from  which  vapor  may  be  formed. 

This  principle  is  utilized  in  the  arts  by  employing  shallow  and 
broad  evaporating  pans.  This  application  is  illustrated  by  the 
process  of  making  salt  from  sea-water.  The  water  is  spread  out  in 
large  pans,  which  are  very  shallow,  and  then  exposed  lo  the  influence 
of  the  sun's  rays,  when  the  water  slowly  evaporates,  leaving  the  salt 
in  the  form  of  crystals. 

Ebullition. 

2*27.  EBULLITION,  or  BOILING,  is  a  rapid  evaporation,  in 
which  the  vapor  escapes  in  the  form  of  bubbles.  The 

What  effect  has  pressure  ?  ffow  is  this  utilised  ?  What  effect  has  change  of  air  ? 
Application  of  this  principle  f  Whnt  effect  has  the  extent  of  liquid?  Hmo  util< 
ized  in  the  arts  ?  Example.  (22  7.)  What  is  Ebullition  ? 


230 


POPULAR     PHYSICS. 


bubbles  are  formed  in  the  interior  of  the  liquid,  and  rising 
to  the  surface,  they  collapse,  permitting  the  vapor  to  pass 
into  the  air. 

In  heating  water,  the  first  bubbles  are  due  to  the  small  quantities 
of  air  contained  in  the  liquid,  which  expand  and  rise  to  the  surface. 
Afterwards,  as  the 
heat  is  kept  up.  par 
ticles  of  water  are  con 
verted  into  vapor  and 
rise  through  the  li 
quid,  becoming  con 
densed  by  the  colder 
layers  of  water  above 
them.  When  all  of 
the  layers  become 
suitably  heated,  the 
bubbles  are  no  longer 
condensed,  but  rise  to 
the  surface,  and  es 
cape  with  a  commo 
tion  that  we  call  boil 
ing,  as  shown  in  Fig. 
147. 

The  following  are 
the  laws  that  gov 
ern  the  phenomena 
of  ebullition  : 

1.  Under  the  same  pressure,  each  liquid  enters  into 
ebullition  at  a  fixed  temperature. 

The  temperature  at  which  a  liquid  boils  is  called  its  boil 
ing  point.  When  the  barometer  stands  at  30  inches,  the 
boiling  point  of  pure  water  is  212°  F.;  the  boiling  point  of 
ether  is  108°  F.;  the  boiling  point  of  alcohol  is  174°  F.,  and 
the  boiling  point  of  mercury  is  660°  F. 


i-  147. 


Explain  the  phenomena  of  -boiling.    What  is  tlie  first  law  of  ebullition?    Illus 
trate. 


VAPORIZATION.  231 

2.  The  pressure  remaining  the  same,  a  liquid  can  not  be 
heated  higher  than  the  boiling  point. 

For  example,  if  water  be  heated  to  212°,  it  will  begin  to 
boil,  and  no  matter  how  much  heat  may  be  applied,  it  will 
continue  to  boil,  but  will  never  become  hotter  than  212°  ; 
all  the  applied  heat  passes  into  the  vapor  and  becomes 
latent.  It  becomes  latent,  because  it  does  not  heat  either 
the  water  or  the  steam  above  212°.  This  will  be  explained 
hereafter. 

Causes  that  modify  the  boiling  point  of  Liquids. 

228.  The  principal  causes  that  influence  the  boiling 
point  of  liquids,  arc  :  the  presence  of  foreign  bodies,  varia 
tions  of  pressure,  and  the  nature  of  the  vessels  in  which  the 
boiling  is  effected. 

1 .  Presence  of  foreign  bodies. — Matter  in  solution  gener 
ally  raises  the  boiling  point  of  a  liquid.     Thus,  a  solution  of 
salt  does  not  boil  so  readily  as  pure  water.     If,  however, 
the  body  dissolved  is  more  volatile  than  water,  then  the 
boiling  point  is  lowered.      Fatty  matters  combined  with 
water,  raise  its  boiling  point.     Hence  it  is,  that  boiling  soup 
is  hotter  than  boiling  water. 

2.  Variations  of  pressure. — Increase  of  pressure  raises, 
and   diminution   of  pressure  depresses,  the  boiling   point. 
When  the  pressure  is  great,  the  vapor,  in  order  to  escape, 
must  have  a  high  tension,  and  this  requires  a  high  temper 
ature.     When  the  pressure  is  small,  the  reverse  is  the  case. 

This  principle  may  be  illustrated  by  the  apparatus  shown  in 
Fig.  148.  It  consists  of  a  bell-glass,  connected  with  an  air-pump. 
Beneath  the  ulass  is  a  vessel  of  water.  If  the  air  be  exhausted  from 


What  is  the  second  law  ?  Illustrate.  (2? 8.)  What  are  the  principal  causes  that 
modify  the  boiling  point?  What  is  the  effect  of  impurities?  Illustrate  by  examples. 
What  is  the  effect  of  pressure?  Illustrate.  Explain  the  experiment. 


232 


POPULAR     PHYSICS. 


the  bell-glass,  the  water  enters  into  ebullition,  even   at  ordinary 
temperatures.     This  is  because  the  pressure  is  diminished. 


Fig.  148. 


If  it  is  desirable  to  continue  the  ebullition  for  some  time,  an 
arrangement  must  be  made  to  remove  the  vapor  as  fast  as  formed. 
This  can  be  effected  by  placing  a  dish  of  sulphuric  acid  under  the 
bell-glass.  The  acid  absorbs  the  vapor  with  great  avidity.  Further 
more,  there  is  no  increase  of  temperature  in  the  water,  but  on  the 
contrary  the  temperature  continually  falls,  and  the  water  may  even 
be  frozen. 

The  same  principle  may  be  further  illustrated  by  a  little  instru 
ment,  shown  in  Fig.  149,  called  FRANKLIN'S  Pulse  Glass.  It  consists 

Hoic  may  water  le  frozen  by  evaporation  ?    Explain  FEANKLIN'S  Pulse  Gloss. 


VAPORIZATION.  233 

of  a  glass  tube,  bent  twice  at  right  angles,  and  terminating  at  each 
extremity  in  a  bulb,  one  of  which  is  somewhat  larger  than  the  other. 
Before  the  larger  bulb  is  sealed,  a  quantity  of  water  is  introduced, 


Fig.  149. 

sufficient  to  fill  the  smaller  one  and  a  part  of  the  larger  one,  and  this 
is  then  made  to  boil  over  a  spirit  lamp  until  the  air  is  driven  out 
and  the  entire  space  is  filled  with  steam.  When  this  is  effected,  the 
large  bulb  is  hermetically  sealed  by  means  of  a  jet  of  flame,  directed 
across  the  open  end  of  the  tube.  The  space  above  the  water  is  then 
filled  with  steam,  which,  as  the  instrument  cools,  is  reduced  to  a  low 
degree  of  tension.  In  this  state  of  affairs  the  heat  of  the  hand 
applied  to  the  small  bulb  is  sufficient  to  make  the  water  boil,  as 
indicated  in  the  figure. 

3.  Nature  of  the  vessel. — When  the  interior  of  the  vessel 
is  rough,  the  projecting  points  form  centres  for  developing 
vapor,  and  the  boiling  point  is  lower  than  when  the  surface 
is  smooth.  Water  boils  at  a  lower  temperature  in  an  iron 
than  in  a  glass  vessel.  In  fixing  the  boiling  point  of  ther 
mometers,  a  metallic  vessel  should  always  be  employed  to 
boil  the  water  in,  on  account  of  the  fact  just  mentioned. 


What  effect  has  the  nature  of  the  vessel  on  ebullition?    Illustrate. 


234 


POPULAR     PHYSICS. 


Papin's  Digester. 

229.  When  water  is  heated  in  open  vessels,  its  temperature 
can  not  be  raised  beyond  a  certain  limit,  but  in  closed  vessels  both 
the  water  and  its  vapor  may  be  raised  to  very  high  temperatures,  so 
that  the  tension  of  the  vapor  may  reach  several  atmospheres.  The 
instrument  employed  to  show  this  fact  is  called  PAPIN'S  Digester, 
so  called  because  PAPIN  invented  it  for  extracting  the  nutriment 
from  bones. 

It  is  represented  in  Fig  150,  and  consists  of  a  thick  bronze  vessel, 
M,  whose  cover  is  held  in 
place  by  a  screw  passing 
through  a  strong  frame.  To 
avoid  danger  of  explosion, 
the  instrument  is  provided 
with  a  safety-valve,  similar 
to  that  used  in  steam-engine 
boilers.  The  safety-valve 
consists  of  a  valve,  u.  fitting 
closely  over  an  opening  in 
the  cover.  This  valve  is 
held  in  place  by  a  lever,  ah, 
and  a  movable  weight,  p. 
One  end  of  the  lever  is 
fastened  at  a  by  a  hinge- 
joint.  By  moving  the  weight, 
p,  along  the  lever,  we  may 
vary  the  force  with  which. 
the  valve,  w,  is  kept  in 


place. 

Suppose  the  weight,  ;?,  to  Fig.  150 

be  30  Ibs.,  or  2  atmospheres, 

then  if  the  distance  arf,  is  made  equal  to  four  times  the  distance  //  , 
from  the  principle  of  the  lever  the  pressure  upon  the  valve  will  he 
that  of  the  atmosphere  increased  by  120  Ibs.,  that  is,  it  will  be  equal 
to  7  atmospheres,  and  whenever  the  tension  of  the  vapor  within  the 


( 229.)  Wiat  is  PAPIN'S  Digester  t    What  principle  does  it  illustrate  ?    Explain 
its  construction. 


VAPOIilZATlON. 


235 


digester  exceeds  this,  the  valve  will  be  forced  open,  and  a  portion 
of  the  steam  will  escape  with  a  whistling  sound  that  indicates  great 
compression. 

If  the  valve  be  left  open,  the  temperature  can  only  be  raised  to 
212°,  and  we  have  the  phenomena  of  simple  boiling.  If  water  be 
heated  in  a  well  corked  bottle,  the  tension  of  the  vapor  will  finally 
cause  the  cork  to  spring  from  its  place  with  a  loud  explosion. 

It  is  the  high  tension  of  con 
fined  vapors  that  gives  rise  to 
the  explosion  of  steam-boilers. 
Hence  the  necessity  of  con 
structing  them  of  strong  mate 
rials,  and  of  providing  them 
with  proper  safety-valves. 

Measure    of    the    Elastic 
Force  of  Vapor. 

23O.  DALTOX  measured 
the  elastic  force  of  watery 
vapor  at  every  tempera 
ture,  from  32°  F.,  up  to 
212°  F.,  by  means  of  the 
apparatus  shown  in  Fig. 
151. 

This  apparatus  consists 
of  two  barometer  tubes,  A 
and  J5,  filled  with  mercury, 
and  inverted  in  an  iron 
boiler,  also  filled  with  the 
same  liquid.  The  tube,  A, 
contains  mercury  alone, 
whilst  the  tube,  ^,  contains 
a  small  quantity  of  water 

Illustrate  its  use  "by  an  example.  What  causes  explosions  of  steam-boilers  f 
Precautions  to  be  taken.  ( 230.)  Explain  D  ALTON'S  apparatus  for  measuring  the 
tension  of  vapors,  and  the  method  of  using  it. 


236  POPULAR     PHYSICS. 

above  the  mercury.  The  tubes  are  kept  in  place  by  a 
wooden  frame,  placed  in  a  long  glass  cylinder  filled  with 
water.  A  thermometer,  £,  is  plunged  into  the  water  for  the 
purpose  of  determining  its  temperature.  When  heat  is  ap 
plied  to  the  boiler,  the  temperature  of  the  whole  apparatus 
is  raised,  and  the  water  in  the  tube,  jft,  is  converted  into 
vapor,  whose  tension  is  made  known  by  the  difference  of 
level  of  the  mercury  in  the  tubes,  A  and  J2.  This  differ 
ence  is  measured  by  a  scale  attached  to  the  cylinder. 

For  example,  if,  when  the  thermometer  stands  at  158°  F., 
the  difference  of  level  in  the  tubes  is  9  inches,  we  say  that 
the  tension  of  vapor  at  158°  is  9  inches  of  mercury,  or 
4.5  Ibs.,  that  is,  it  presses  each  square  inch  of  surface,  with 
which  it  is  in  contact,  with  a  force  of  4.5  Ibs. 

DALTON  increased  the  temperature  from  32°  to  212°,  noting  at 
each  degree  the  difference  of  level  between  the  mercury  in  the  tubes, 
and  thus  was  enabled  to  form  a  table  showing  the  elastic  force  of 
vapor  at  all  temperatures  within  these  limits. 

DULONTG  and  ARAGO  have  more  recently  extended  DALTON'S  table 
to  temperatures  above  212°.  Their  investigations  show  that  the 
tension  of  watery  vapor  at  212°  F.  is  1  atmosphere;  at  250°  F.  it 
is  2  atmospheres;  at  273°  F.  it  is  3  atmospheres;  at  291°  F.  it  is 
4  atmospheres ;  at  306°  F.  it  is  5  atmospheres. 

From  all  of  these  results  we  infer  that  the  tension  increases  very 
rapidly  with  the  temperature. 

Latent  Heat  of  Vapors. 

231.  When  a  liquid  begins  to  boil,  all  of  the  heat  that 
is  added  enters  into  the  vapor  and  becomes  latent.  The 
amount  of  heat  that  becomes  latent,  is  different  for  different 
liquids.  It  is  called  the  latent  heat  of  vaporization. 

It  has  been  ascertained  by  experiment  that  the  latent  heat  of 
watery  vapor  is  about  990°  F.,  that  is,  it  takes  5J  times  as  much 

Example.  Between  what  limits  does  DALTON'S  table  extend  ?  What  general 
inference  maybe  drawn?  (231.)  What  is  latent  heat  of  vaporization?  What 
does  it  amount  to  for  water  f 


VAPORIZATION.  237 

heat  to  convert  any  quantity  of  water  into  steam  as  is  required  to 
raise  the  same  quantity  of  water  from  the  freezing  to  the  boiling 
point.  This  may  be  verified  by  mixing  1  Ib.  of  steam  at  212°  with 
51  Ibs.  of  water  at  32°.  The  latent  heat  becomes  sensible  by  the 
condensation  of  the  vapor,  and  there  results  6i  Ibs.  of  water  at  212°. 


Examples  of  Cold  produced  by  Heat  becoming  Latent. 

232.  If  a  few  drops  of  ether  be  poured  upon  the  hand  and 
allowed  to  evaporate,  a  sensation  of  cold  will  be  felt.  The  ether  in 
evaporating  extracts  the  heat  from  the  hand,  which  becomes  latent. 

Damp  linen  feels  cold  when  applied  to  the  body,  because  the 
moisture  in  passing  to  a  state  of  vapor  extracts  the  animal  heat, 
which  entering  the  vapor,  becomes  latent: 

The  warm  wind  of  summer  is  refreshing,  because  it  causes  a  more 
rapid  evaporation  of  the  perspiration,  which  abstracts  animal  heat 
from  the  body  to  become  latent  in  the  vapor  thus  produced.  The 
coolness  that  results  from  sprinkling  the  floor  of  an  apartment  in 
summer,  arises  from  the  passage  of  heat  from  a  sensible  to  a  latent 
state,  in  consequence  of  the  evaporation  of  the  water.  For  the  like 
reason,  a  shower  of  rain  is  generally  followed  by  a  diminished  tem 
perature. 

Water  may  be  cooled  by  putting  it  in  porous  vessels.  A  small 
quantity  escapes  through  the  pores,  and  in  evaporating  abstracts  a 
portion  of  heat  from  the  remaining  liquid,  thus  reducing  its  temper 
ature.  This  is  the  process  of  cooling  water  employed  in  many 
tropical  countries. 


Congelation  of  Water  and  Mercury  in  a  Vacuum. 

233.  "When  evaporation  is  rapidly  increased,  the  ab 
sorption  of  heat  is  proportionally  increased,  and  as  it  is 
taken  from  the  surrounding  objects,  these  are  sometimes 
frozen.  It  has  been  stated  that  water  may  be  frozen  under 

How  is  this  shown?  (232.)  Why  does  ether  produce  cold  ~by  evaporation? 
Why  does  damp  linen  feel  cold  ?  Why  is  warm  wind  refreshing  in  summer  ? 
Effect  of  sprinkling  ?  Of  a  shower  ?  How  is  water  cooled  in  porous  vessels  ? 
(  233.)  Why  does  evaporation  produce  cold  in  surrounding  objects  ? 


238  POPULAR   PHYSICS. 

the  receiver  of  the  air-pump  by  absorbing  the  vapor  as 
rapidly  as  it  is  generated. 

By  operating  with  a  liquid  more  volatile  than  water,  a 
greater  degree  of  cold  is  produced.  By  using  sulphurous 
acid,  which  boils  at  14°  F.,  a  sufficient  degree  of  cold  is  pro 
duced  to  freeze  mercury.  This  is  effected  by  surrounding 
a  thermometer  bulb  with  cotton,  saturated  with  sulphurous 
acid,  and  then  placing  it  under  a  receiver  and  exhausting 
the  air. 

The  rapid  vaporization  abstracts  so  much  heat  from  the 
mercury  that  it  freezes  in  a  few  minutes.  If  we  break  the 
bulb,  the  mercury  is  found  in  a  solid  mass  like  a  leaden 
bullet.  In  this  form  mercury  can  be  drawn  out  into  sheets, 
or  stamped  like  a  coin,  but  it  soon  absorbs  heat  from  neigh 
boring  bodies,  and  again  passes  to  a  liquid  state 


VIII. —  CONDENSATION     OF     GASES     AND     VAPORS. —  SPECIFIC     HEAT. 

Causes  of  Condensation. 

234.  The  CONDENSATION  of  a  vapor,  is  its  change  from 
a  vaporous  to  a  liquid  state.  This  change  of  state  may  arise 
from  chemical  action,  pressure,  or  diminution  of  temper 
ature. 

1.  Chemical  action. — The  affinity  of  certain  substances 
for  the  vapor  of  water  is  so  strong  that  they  absorb  it  from 
the  air,  even  when  the  latter  is  not  saturated;  such,  for 
example,  are  quick-lime,  potash,  sulphuric  acid,  and  many 
others.     When  placed  in  a  closed  space,  they  in  a  short  time 
abstract  all  of  the  moisture  that  is  in  it. 

2.  Pressure. — If  a  closed  cylinder  be  filled  with  vapor,  and 

Explain  the  experiment  with  sulphurous  acid.  Can  mercury  be  frozen?  (234.) 
What  is  condensation  of  a  vapor ?  Causes?  Effect  of  chemical  action ?  Examples. 
Effect  of  pressure? 


CONDENSATION     OF    YAPOES.  230 

this  be  compressed  by  a  piston,  as  soon  as  the  space  occu 
pied  by  the  vapor  is  saturated,  it  will  begin  to  condense, 
and  if  the  pressure  be  continued,  all  the  vapor  will  be 
reduced  to  the  liquid  state.  Until  the  space  becomes  satu 
rated,  the  pressure  must  be  continually  increased,  on  account 
of  the  augmented  tension  of  the  vapor,  but  after  liquefaction 
begins,  no  further  augmentation  of  tension  takes  place,  and 
the  pressure  required  to  complete  the  liquefaction  remains 
uniform. 

3.  Diminution  of  temperature. — When  the  temperature 
of  any  space  is  diminished,  the  amount  of  vapor  required  for 
saturation  is  diminished.  After  the  point  of  saturation  is 
reached,  any  further  diminution  of  temperature  causes  a 
deposit  of  the  vapor  in  a  liquid  form. 

Steam  is  colorless,  but  when  allowed  to  escape  into  the  cold  air, 
condensation  takes  place  in  the  form  of  drops,  which  become  visible. 
For  the  same  reason,  the  moisture  contained  in  the  breath  becomes 
visible  in  cold  weather. 

In  winter  the  glass  of  our  windows  often  becomes  coated  with 
drops  like  dew.  This  arises  from  the  fact  that  the  glass  is  colder 
than  the  air  of  the  room,  and  thus  acts  continually  to  produce  con 
densation  of  the  vapor  in  the  air.  If  the  difference  of  temperature  is 
sufficient,  the  particles  of  vapor  are  frozen  as  they  are  deposited, 
producing  beautiful  crystallizations.  When  the  external  air  is 
warmer  than  that  within,  the  deposit  takes  place  on  the  outside  of 
the  glass.  If  a  vessel  of  cold  water  be  placed  in  a  warm  room,  a 
deposition  of  moisture  takes  place  on  its  exterior  surface. 

The  nearer  the  air  is  to  saturation,  the  more  abundant  is  the 
deposit  of  dew.  Hence,  before  a  rain,  the  deposit  is  especially 
abundant.  Stone  walls,  and  the  like,  being  cooler  than  the  atmos 
phere,  are  often  in  summer  covered  with  moisture,  when  they  are 
said  to  sweat.  The  moisture  in  this  case  is  condensed  from  the  air, 

Illustrate.  How  long  nwst  the  pressure  augment?  Effect  of  diminution  of  tem 
perature  ?  What  is  the  color  of  steam,  ?  Why  does  it  become  visible  f  Explain  the 
deposition  of  drops  on  glass.  Explain  frost-work  crystals.  Why  is  the  deposition 
bundant  before  rain  f  Deposition  on  stones  and  walls? 


POPULAR     PHYSICS. 

and  does  not  come  from  the  stones.  If  the  sweating  of  stones  is 
indicative  of  rain,  it  is  because  the  deposition  is  most  abundant  when 
the  air  is  most  nearly  saturated. 

Heat  developed  by   Condensation. 

235.  When  a  liquid  passes  to  a  state  of  vapor,  a  great 
quantity  of  heat  is  absorbed  from  neighboring  bodies,  and 
becomes  latent.     When  the  vapor  returns  to  a  liquid  state, 
an  equal  amount  of  heat  is  given  out  and  becomes  capable 
of  affecting  our  senses ;  in  other  words,  it  becomes  sensible. 

Heating  by   Steam. 

236.  Buildings  are  heated  by  means  of  steam  conveyed 
from  a  boiler  in  the  lower  story,  through  iron  pipes  in  the 
Avails.     The  steam,  by  its  heat  and  by  the  heat  given  out  on 
condensation,  serves  to  warm  the  apartments  through  which 
it  is  made  to  pass.     To  this  end,  coils  of  pipes  are  placed  in 
the  rooms  to  be  warmed. 


Distillation. 

237.  DISTILLATION  is  the  process  of  separating  liquids 
from  each  other  by  means. of  heat. 

The  most  volatile  of  the  liquids  is  most  easily  evaporated, 
and  its  vapor  is  then  condensed.  The  heat  should  be  kept 
above  the  boiling  point  of  the  liquid  that  we  wish  to  obtain^ 
but  below  that  which  we  wish  to  leave  behind.  The  boiling 
point  of  alcohol  being  174°  F.,  and  that  of  water  212°,  if  a 
mixture  of  alcohol  and  water  be  heated  up  to  some  tem 
perature  between  these  limits,  the  alcohol  will  all  be  vapor 
ized,  whilst  most  of  the  water  will  remain  behind. 


Why  indicative  of  rain  f  (235.)  Explain  the  development  of  heat  by  conden 
sation  ?  ( 236.)  Explain  the  principle  of  heating  buildings  by  steam?  (237.)  What 
is  distillation  ?  "What  degree  of  heat  is  required  for  distillation  ? 


CONDENSATION     OF     VAPOKS. 


Distillation. 

An  ALEMBIC,  or  Still,  is  an  apparatus  for  distilla 


tion. 


The  most  usual   form  of  an   alembic  is  represented  in 
ig.  152.     It  is  composed  of  a  boiler,  A.,  with  a  cover,  7>, 


Fig.  152 

called  the  dome  ;  from  the  top  of  the  dome  a  metallic  tube, 
(7,  passes  into  a  vessel,  8,  called  the  condenser,  and  is  then 
bent  into  a  spiral  form.  This  tube  is  called  the  ivorm,  and 
after  passing  through  the  condenser,  8,  it  leads  to  a  receiver, 
D.  The  condenser,  8,  is  kept  full  of  cold  water  by  an 
arrangement  shown  in  the  figure. 

The  substance  to  be  distilled  is  placed  in  A,  and  a  suitable 
heat  is  then  applied.     The  more  volatile  portion  is  converted 


(238.)  What  is  an  Alemlic  ?    Describe  the  most  usual  form  ?    How  is  distillation 
effected  ? 

11 


24:2  POPULAR     PHYSICS. 

into  vapor,  rises  into  the  dome,  and  passing  through  the 
worm,  is  condensed,  and  escapes  in  a  liquid  form  into  the 
receiver,  D. 

Wine  is  composed  of  water,  alcohol,  and  a  coloring  matter.  If 
this  liquid  be  placed  in  the  alembic  and  heated  to  any  temperature 
between  174°  and  212°,  the  alcohol  is  separated  from  the  other 
ingredients.  As  a  portion  of  water  is  evaporated,  the  alcohol  thus 
obtained  is  not  pure,  and  will  require  to  be  distilled  again.  At 
each  distillation,  the  strength  is  increased,  but  no  amount  of  distilla 
tion  can  render  it  absolutely  pure. 

By  distillation,  pure  water  may  be  obtained  from  the  brine  of  the 
ocean,  or  from  the  impure  water  of  our  wells  and  springs. 


Liquefaction   of  Gases. 

239.  Most  of  the  gases  have  been  liquefied,  either  by 
pressure  alone,  or  by  a  combination  of  pressure  with  a 
diminution  of  temperature.  An  immense  pressure  may  be 
had  by  utilizing  the  tension  of  the  gases  themselves,  by 
generating  large  quantities  in  confined  spaces. 

One  of  the  most  interesting  examples  of  the  liquefaction  of  a  gas  is 
that  of  carbonic  acid. 

Carbonic  acid  is  capable  not  only  of  liquefaction,  but  also  of  con 
gelation.  For  this  purpose,  two  immensely  strong  cylinders  are 
fitted  together,  both  being  hermetically  sealed,  and  communicating  by 
a  pipe.  One  of  these  cylinders  is  the  generator,  and  the  other  the 
receiver.  In  the  generator  are  placed  the  ingredients  necessary  to 
generate  carbonic  acid,  usually  carbonate  of  soda  and  sulphuric  acid. 
After  the  opening  is  carefully  closed,  these  materials  are  brought 
into  contact,  when  an  immense  volume  of  carbonic  acid  is  developed, 
and.  being  unable  to  expand,  its  tension  becomes  so  great  that  a  por 
tion  is  condensed  into  a  liquid  form.  The  tension,  at  the  temperature 
of  60°  F.,  is  equal  to  50  atmospheres,  or  750  Ibs.  on  each  square 
inch. 

Explain  the  method  of  distilling  alcohol  ?  Water?  (  239.)  How  may  pases  be 
liquefied?  Example.  Explain  the  apjpasfJusfor  liquefying  cirbpnic  jzcid  f  Tht> 
process  of  liquefaction  t 


SPECIFIC     HEAT.  24:3 

After  liquefaction  has  ceased,  if  a  stop-cock  be  turned  so  as  to 
allow  a  part  of  the  confined  gas  to  escape,  a  portion  of  the  liquid 
acid  passes  to  a  state  of  vapor  with  immense  rapidity,  and  in  doing 
so,  absorbs  so  much  heat  from  the  remaining  portion  as  to  freeze  it. 
The  frozen  acid  is  thrown  out  by  the  gaseous  jet  in  flakes  like  snow. 
It  is  very  white,  and  so  cold  as  to  freeze  mercury  instantly.  It 
evaporates  very  slowly,  and  when  tested  with  a  spirit  thermometer, 
its  temperature  is  found  to  be  112°  below  the  0  of  FAHRENHEIT'S 
thermometer.  By  using  this  solid  with  other  substances  for  which  it 
has  an  affinity,  the  greatest  degree  of  artificial  cold  may  be  obtained. 

Specific  Heat   of  Solids   and  Liquids. 

24O.  Experiment  shows  that  different  bodies  require 
different  amounts  of  heat  to  elevate  their  temperatures 
through  the  same  number  of  degrees.  The  amount  of  heat 
required  to  heat  any  body  a  certain  number  of  degrees,  is 
called  its  specific,  heat. 

If  equal  weights  of  water,  iron,  and  mercury  have  the 
same  amount  of  heat  communicated  to  them,  the  mercury 
will  be  most  heated,  the  iron  next,  and  the  water  least  of 
all.  When  heated  to  a  certain  temperature,  water  absorbs 
ten  times  as  much  heat  as  iron,  and  thirty-three  times  as 
much  as  mercury. 

In  order  to  compare  bodies  with  respect  to  their  specific 
heat,  we  take  as  a  unit  the  amount  of  heat  necessary  to  raise 
a  given  weight,  say  1  lb.,  of  water  through  1°  F.  Two 
principal  methods  have  been  employed  to  ascertain  the 
relative  specific  heat  of  bodies. 

In  the  first  method,  the  body  to  be  experimented  upon  is 
brought  to  a  standard  temperature,  say  212°  F.,  and  is  then 
brought  into  contact  with  ice.  The  amount  of  ice  melted 
makes  known  the  quantity  of  heat  given  off  by  the  body  in 

How  may  a  portion  be  solidified  t  Describe  the  solid  gas.  ffow  may  intense 
cold  ~be  produced?  What  degree  of  Fahrenheit?  (240.)  What  is  specific  heat? 
Illustrate.  How  do  we  compare  bodies  with  respect  to  specific  heat  ?  Explain  the 
first  method  of  determining  the  specific  heat  of  a  body. 


244: 


POPULAR     PHYSICS. 


passing  from   212°  to  32°,  from  which  the  relative  specific 
heat  may  be  determined. 

In  the  second  method,  the  body  to  be  experimented  upon 
is  heated  to  a  certain  temperature,  and  then  plunged  into 
water  at  a  lower  temperature.  The  two  bodies  interchange 
heat  and  come  to  a  common  temperature.  Then,  from  a 
knowledge  of  the  weights  of  the  two  bodies  mixed,  their 
original  temperatures,  and  their  common  resulting  temper 
ature,  their  relative  specific  heats  may  be  determined. 

The  following  table  shows  the  specific  heat  of  a  few  of  the  most 
important  substances  : 

TABLE. 


SUBSTANCE. 

SPECIFIC   HEAT. 

SUBSTANCE. 

SPECIFIC  HEAT. 

Water 

1  000 

Copper 

0  095 

Glass  

0.198 

Silver 

0.057 

Iron  

O.i  14 

Mercury 

0.033 

Zinc  

0.096 

Platinum  

0.032 

Of  all  these  bodies  water  has  the  greatest  specific  heat, 
and  consequently  it  requires  more  heat  to  raise  its  temper 
ature  through  any  given  number  of  degrees. 

Water  heats  slowly,  and  mercury  very  rapidly.  Of  course 
mercury  cools  rapidly  and  water  slowly. 

The  specific  heats  of  gases  have  been  determined  with  respect  to 
air  as  a  standard,  but  the  results  need  not  be  given  in  this  treatise. 


The  second  method.    What  body  has  the  greatest  specific  heat  ?    What  bodies  heat 
fastest  ?    Cool  fastest  ?    Examples. 


HYGROMETRY.  245 

IX.—  HYGROMETRY.  —  RAIN.  —  DEW.  —  WINDS. 

Hygrometry. 

241.  HYGROMETRY  is    the    process   of  measuring  the 
amount  of  moisture  in  the  air  with  respect  to  the  amount 
necessary  to  saturate  it. 

The  object  of  hygrometry  is  not  to  determine  the  absolute  amount 
of  moisture  in  the  atmosphere,  but  simply  to  find  out  its  degree  of 
saturation.  The  absolute  amount  of  moisture  remaining  the  same, 
the  atmosphere  might  at  one  temperature  be  saturated,  whilst  at 
some  other  temperature  it  would  be  far  from  saturation. 

In  winter  the  air  is  generally  damper  than'  in  summer, 
though  in  the  latter  season  it  generally  contains  a  greatet 
absolute  amount  of  vapor  than  in  the  former.  This  is  due 
to  difference  of  temperature.  For  the  same  reason  the  ah' 
is  damper  at  night  than  in  the  day  time.  A  cold  room  is 
damper  than  a  warm  one  for  the  same  reason. 

Moisture  in  the   Air,   and  its   Effects. 

242.  The  quantity  of  moisture  in  the  air  varies  with  the 
seasons,  with  the  temperature,  with   the  climate,  and  with 
different  local  causes. 

When  the  air  is  too  dry,  the  exhalation  by  the  pores  of  the  skin, 
called  insensible  perspiration,  is  too  abundant,  the  skin  cracks,  and 
exfoliates,  and  much  suffering  results.  When  the  air  is  too  moist, 
the  insensible  perspiration  is  retarded  and  often  entirely  stopped, 
resulting  in  many  painful  diseases. 

Hence  the  importance,  in  a  sanitary  point  of  view,  of  regulating 
the  amount  of  moisture  in  our  dwellings  so  as  to  avoid  both  of  theso 
extremes.  On  this  account  it  is  that  evaporators  are  attached  to  our 

(241.)  What  is  Hygrometry?  Illustrate.  Explain  the  difference  between  the 
hysrrometrical  state  of  the  air  in  winter  and  summer.  (  242.)  Under  what  circum 
stances  does  the  quantity  of  moisture  in  the  air  vary?  Explain  the  effect  ofdryne  s 
and  moisture  on  the  system.  Important  sanitary  precaution. 


246 


POPULAR    PHYSICS. 


furnaces,  which,  when  properly  regulated,  keep  up  a  suitable  degree 
of  moisture  in  the  heated  air,  furnished  to  warm  our  apartments. 

The  Hygroscope. 

243.  A  HYGKOSCOPE  is  an  instrument  for  showing  the 
amount  of  moisture  in  the  air. 

Any  hygrometric  substance,  that  is,  any  substance  capable 
of  absorbing  moisture,  may  be  employed  as  a  hygroscope. 
A  great  number  of  animal  and  vegetable  substances,  such 
as  paper,  parchment,  hair,  catgut,  are  elongated  by  absorb 
ing  moisture,  and  are  shortened  when  dried,  and  are  there 
fore  adapted  to  the  construction  of  a  hygroscope.  We  shall 
explain  the  construction  of  a  single  instrument  of  this  class 
in  illustration  ot  the  principle  employed  in  all. 

It  consists,  as  shown  in 
Fig.  153,  of  a  piece  of  wood 
cut  out  in  the  shape  of  a 
monk,  having  a  cowl  of 
pasteboard  turning  about 
an  axis,  a.  The  axis,  a, 
passes  through  the  neck  of 
the  figure,  and  connects 
with  an  apparatus  shown 
in  the  section  AB,  on  the 
left  of  the  figure.  The  axis, 
a,  is  connected  with  a  piece 
of  twisted  catgut  kept  tense 
by  a  spring.  When  the 
weather  is  dry,  the  catgut 
twists  tighter,  carrying  with 
it  the  axis  a,  and  the  monk 
lays  off  his  cowl,  as  shown 
in  the  figure.  When  the  weather  is  damp,  the  catgut  ui> 

(  243  )  What  is  a  Hygroscope  ?    What  substances  may  be  used  in  the  construction 
of  a  hygroscope  ?    Examples.    Explain,  the  hygroscope  shown  in  Fig.  158. 


HVvillOilJLTJIY. 


247 


twists,  and  the  monk  puts  on  his  cowl.  In  adjusting  the 
instrument,  care  should  be  taken  to  have  the  cowl  on  the 
head  when  the  catgut  is  damp. 

Instruments  of  this  kind  are  very  uncertain  in  their  action,  and 
are  therefore  used  as  matters  of  curiosity  rather  than  for  any  scientific 

value  they  may  possess. 

:( 
The  Hair  Hygrometer. 

244.     A  HYGROMETER  is  an  instrument  for  measuring 
the  amount  of  moisture  in  the  air. 
Several  kinds  have  been  invented; 
but  the  hair  hygrometer  is  the  most 
used. 

This  instrument  is  constructed 
from  the  principle  that  a  hair  elon 
gates  when  moistened,  and  shortens 
when  dried.  The  form  usually  given 
to  it  is  shown,  in  Fig.  154.  A  hair 
about  eight  inches  in  length  is  fast 
ened  at  its  upper  end,  and  at  its 
lower  end  it  is  wound  around  the 
axis  of  a  small  pulley,  and  then  is 
made  fast  to  it.  A  silk  thread  is 
wound  around  the  pulley  in  an  op 
posite  direction,  having  a  weight, 
P,  attached  to  it  to  keep  it  tense. 
A  needle  attached  to  the  pulley 
plays  in  front  of  a  graduated  arc,  as 
the  hair  elongates  and  contracts.  Fig.  154. 

To  graduate  the  instrument,  it  is  placed  under  a  bell-glass,  and 
the  air  is  thoroughly  dried  by  some  substance,  such  as  quick-lime, 
which  is  capable  of  absorbing  the  moisture  of  the  air.  The  point  at 
which  the  needle  then  stands  is  marked  0.  The  air  is  then  saturated 


(244.)  What  is  a  Hygrometer?     Explain  the  construction  and  use  of  the  hair 
hygrometer.    How  is  it  graduated  t 


248  POPULAR     PHYSICS. 

with  moisture,  and  the  point  at  which  the  needle  stands  is  marked 
100.  The  intervening  space  is  divided  into  100  equal  parts,  and 
these  are  numbered  from  0  up  to  100.  The  temperature  is  noted  by 
a  thermometer  attached  to  the  frame  of  the  instrument. 

To  use  the  instrument,  we  note  the  reading  of  the  needle 
and  of  the  thermometer,  and  from  these  the  exact  amount 
of  moisture  in  the  air  may  bj  computed. 

Hygrometric   state  of  the   Atmosphere. 

245.  By  the  HYGROMETRIC  STATE  of  the  atmosphere,  we 
mean  its  relative  degree  of  saturation.     If  we  denote  com 
plete  saturation  by  1,  and  the  air  contain    half  the  amount 
of  vapor  necessary  to  saturate  it,  its  hygrometric  state  will 
be  denoted  by  0.5. 

GAY  LUSSAC  has  constructed  a  table,  by  means  of  which  the 
hygrometric  state  of  the  air  may  be  found  when  we  know  the  read 
ing  of  the  hygrometer  already  described,  together  with  that  of  the 
attached  thermometer. 

Formation  of  Fogs   and   Clouds. 

246.  FOGS  and  CLOUDS  are  masses  of  vapor  condensed 
into  drops,  or  vesicles,  by  coming  in  contact  with  colder 
strata  of  the  atmosphere.     The  term  fog,  applies  when  these 
masses  are  in   contact  with  the  earth,  and  the  term  cloud, 
when  they  are  suspended  in  the  air. 

The  air  at  all  times  contains  a  greater  or  less  quantity  of 
invisible  vapor,  and  if  at  any  time  the  air  becomes  cooled 
below  a  certain  limit,  a  portion  is  condensed  and  becomes 
visible  ;  the  result  is  either  a  fog  or  a  cloud. 

One  of  the  most  common  causes  of  clouds  is  the  cold  generated  by 
nn  ascending  current  of  air.  When  the  air  becomes  heated,  it  ex- 


How  is  it  used?  (245.)  What  is  meant  by  the  hygrometric  state  of  the  atmos 
phere?  (246.)  What  are  Fogs?  Clouds?  How  are  fogs  and  clouds  formed? 
What  is  a  common  cause  of  a  cloud  f 


KAIN,     DEW,     AND     FROST.  249 

pands  and  ascends,  and  being  continually  subjected  to  a  diminishing 
pressure,  it  expands  rapidly,  and  a  large  amount  of  heat  must  become 
latent.  This  absorption  of  heat  produces  cold  enough  to  condense 
tlic  vapor  into  clouds.  When  a  cloud  floats  into  a  warmer  stratum 
of  the  atmosphere,  it  is  often  converted  into  invisible  vapor  and  dis 
appears.  It  is  dissolved. 

Mountains  arrest  the  winds  blowing  from  the  plains,  and  force 
them  to  ascend  their  sloping  sides.  Coming  in  contact  with  the 
colder  strata  of  the  atmosphere,  the  moisture  is  converted  into  clouds 
and  fogs.  Hence  we  often  see  the  mountain  tops  covered  with  fogs 
and  clouds,  when  the  other  portions  of  the  sky  are  clear.  The  con 
densation  of  water  on  the  sides  of  mountains  is  the  most  fruitful 
source  of  our  streams.  When  a  cold  wind  meets  with  a  warm 
and  moist  current  of  air,  the  cooling  process  is  so  great  as  to  generate 
clouds. 

Two  theories  have  been  advanced  to  explain  the  reason 
why  clouds  remain  suspended  in  the  air.  According  to  the 
first  theory,  the  particles  of  moisture  are  hollow  spheres  of 
water  like  soap-bubbles,  filled  with  air  less  dense  than  that 
without.  Consequently  the  little  vesicles  float  in  the  air 
like  so  many  minute  balloons.  According  to  the  second,  and 
favorite  theory,  the  particles  are  extremely  small,  and  float 
in  the  air  in  the  same  way  that  particles  of  dust  and  other 
small  bodies  are  seen  to  be  borne  along  by  the  atmos 
phere. 

Fogs  form  over  bodies  of  water  and  moist  grounds,  when  the  air 
above  them  is  cooler  than  the  water  or  earth. 

Fogs  are  frequent  along  the  course  of  rivers  and  upon  inland 
lakes.  The  cause  of  the  dense  fogs  that  prevail  in  the  neighborhood 
of  Newfoundland,  is  the  Gulf  Stream.  The  water  brought  by  the 
Gulf  Stream  is  warmer  than  that  of  the  surrounding  ocean,  and  as 
the  vapor  rises  from  it,  it  is  converted  by  the  cold  air  from  the 
neighboring  regions  into  fog. 


When  does  a  cloud  dissolve  ?  Effect  of  mountains  on  clouds  ?  Utility  of  moun 
tain  condensation  f  Explain  the  two  theories  of  the  formation  of  clouds.  Where 
are  fogs  most  frequent  ?  Why  so  many  fogs  on  the  banks  of  Neiofoundland  f 

11* 


250  POPULAR     PHYSICS. 


Rain. 

247.  RAIN  is  a  fall  of  drops  of  water  from  the  atmos 
phere.  When  several  particles  of  a  cloud  unite,  the  weight 
becomes  too  great  to  be  supported  by  the  air,  and  the  drop 
thus  formed  falls  to  the  ground. 

When  a  cloud  floats  into  a  colder  stratum  of  the  atmosphere,  it 
becomes  more  condensed,  and  we  have  a  fall  of  rain.  When  it  floats 
into  a  warmer  stratum  it  dissolves.  Hence  we  often  see  the  clouds 
of  the  morning  dissolve  under  the  influence  of  the  sun,  which  acts  to 
heat  the  upper  regions  of  the  atmosphere. 

The  quantity  of  rain  that  falls  in  any  country  depends 
upon  its  neighborhood  to  the  ocean  or  other  bodies  of 
Water,  upon  the  season,  upon  the  temperature,  and  upon  the 
prevailing  direction  of  the  winds.  More  rain  falls  near  the 
coasts  than  in  the  interior ;  more  rain  falls  in  summer  than  in 
winter ;  more  rain  falls  in  tropical  climates  than  in  temper 
ate  and  polar  climates  ;  and  finally,  more  rain  falls  in  those 
countries  where  the  prevailing  winds  are  from  the  ocean 
than  where  they  are  from  the  continents. 

The  following  table  indicates  the  number  of  inches  of  rain  that 
fall  during  the  year  at  the  places  named  : 

At  Copenhagen 18  inches. 

"    Paris 22       " 

"    Havana 90      " 

u    Calcutta 81       " 

"   Grenada 126      '•'• 

From  this  we  see  that  the  quantity  of  rain  increases  rapidly  as  we 
approach  the  equatorial  regions. 


( 247.)  What  is  Eain  ?  Explain  the  cause  of  rain.  Upon  what  does  the  amount 
of  rain  in  anyplace  depend?  Give  examples  of  the  amount  of  rain  in  different 
places.  Inference. 


RAIN,     DEW,     AND    FROST.  251 


Dew  and    Frost. 

248.  DEW  is  a  deposition  of  watery  particles,  that  takes 
place  upon  the  soil  and  plants  during  the  calm  nights  of 
summer. 

The  true  theory  of  dew  was  first  established  by  WELLS. 
According  to  his  theory,  dew  results  from  the  earth  and 
plants  becoming  cooled  by  radiation,  thus  producing  a  de 
posit  of  moisture  from  the  neighboring  strata  of  air.  Good 
radiators  are  soonest  covered  with  dew,  whilst  bad  radiators 
have  little  or  no  dew  formed  upon  them. 

The  state  of  the  atmosphere  influences  the  amount  of  dew. 
When  the  air  is  clear,  the  dew  is  abundant,  when  cloudy, 
little  or  no  dew  is  formed.  In  this  case  the  clouds  radiate 
heat  to  the  earth,  and  this  prevents  the  latter  from  cooling 
so  rapidly.  A  strong  breeze  prevents  the  formation  of  dew, 
by  removing  the  strata  of  air  next  the  earth  before  they 
have  time  to  be  cooled  down  to  the  point  of  saturation,  or 
the  dew  point.  A  gentle  breeze  may  facilitate  the  forma 
tion  of  dew,  by  replacing  the  layer  of  air  from  which  the 
water  has  been  deposited,  by  another  which  contains  more 
moisture. 

WHITE  FROST  is  nothing  more  than  frozen  dew.  It  is 
often  seen  in  autumn,  and  arises  under  the  same  circum 
stances  as  are  favorable  to  the  formation  of  dew.  In  order 
that  frost  may  occur,  the  earth  must  be  cooled  below  32°  F. 

Snow   and  Hail. 

249.  SNOW  is  a  collection  of  frozen  particles  of  water, 
formed  in  the  upper  regions  of  the  atmosphere,  whence  it 
falls  to  the  ground  in  ftakej. 


(248.)  "What  is  Dew  ?  What  is  WELLS'  theory  of  dew  ?  What  bodies  are  soonest 
covered  with  dew?  What  ones  have  little  dew  upon  them?  What  effect  has  the 
state  of  the  atmosphere  on  dew  ?  Why  is  there  much  dew  on  clear  nights  ?  Little 
on  cloudy  nights?  What  is  the  dew  point?  Effect  of  a  gentle  breeze?  What  is 
White  Frost  ?  (  249.)  What  is  Snow  ? 


252  POPULAR  PHYSICS. 

Snow  flakes  are  made  up  of  crystals,  arranged  in  star-like 
forms  with  three  or  six  branches,  differently  arranged,  but 
always  remarkable  for  their  regularity  and  beauty.  When 
snow  falls,  the  temperature  of  the  air  is  near  32°  F.  If  the 
temperature  is  much  lower,  the  snow  is  less  abundant,  be 
cause  the  amount  of  vapor  in  the  air  is  less. 

The  quantity  of  snow  that  falls  in  any  place  is  generally  the 
greater  as  the  place  is  nearer  the  pole,  or  as  it  is  higher  above  the 
level  of  the  ocean.  At  the  poles,  and  on  the  summits  of  high 
mountains  in  all  latitudes,  snow  remains  through  the  entire  year. 
As  we  approach  the  equator,  the  region  of  perpetual  snow  rises 
higher  and  higher  above  the  level  of  the  ocean.  In  the  Andes,  under 
the  equator,  the  limit  of  perpetual  snow  is  between  15.000  and 
16,000  feet  above  the  level  of  the  ocean;  in  the  Alps  it  is  only 
10.500  feet  above  the  level  of  the  ocean;  towards  the  northern 
extremity  of  Norway  it  is  but  3.000  feet  above  the  ocean  level. 

HAIL  is  composed  of  layers  of  compact  ice,  arranged  con 
centrically  about  nuclei  of  snow.  Its  formation  is  undoubt 
edly  of  electrical  origin,  and  will  be  again  treated  of  under 
the  head  of  electricity. 

Winds. 

250.  WIXDS  arc  currents  of  air,  moving  with  greater 
or  less  rapidity.     They  are  generally  named  from  the  quarter 
whence  they  blow ;  thus  a  wind  that  blows  from  the  east 
is  called  an  east  wind,  and  so  for  other  winds.     Winds  are 
sometimes  named  from  some  local  peculiarity.     Thus  we 
have  trade  winds,  monsoons,  siroccos,  and  the  like.      The 
prevailing  directions  of  the  wind  are  different  in  different 
countries,  for  reasons  that  will  be  explained  hereafter. 

Causes   of  Winds. 

251.  Winds  are  caused  by  variations  of  temperature  in 
the  atmosphere  ;    these  variations  produce  expansions  and 

Describe  a  snow  flake.     WJiat  laio  governs  the,  fall  of  snow?    What  is  Hail? 
(2  SO.)  What  are  Winds?    How  named?     (  251.)  What  are  the  causes  of  winds? 


WINDS.  253 

contractions,  thus  disturbing  the  equilibrium  of  the  atmos 
phere,  causing  currents.  These  currents  are  winds  For 
example,  if  the  air  is  more  heated  over  one  country  than 
over  the  neighboring  countries,  it  dilates  and  rises,  its  place 
being  supplied  by  the  colder  air  which  flows  in  from  the 
surrounding  regions.  The  surplus  of  air  thus  brought  in 
flows  over  at  the  top  of  the  ascending  column.  Hence 
there  is  a  current  near  the  earth  in  one  direction,  whilst  at 
a  higher  elevation  there  is  a  current  flowing  in  a  contrary 
direction. 

Regular,  Periodic,  and  Variable  Winds. 

252.  Winds  are  divided  into  three  classes :  REGULAR 
WINDS,  PERIODIC  WINDS,  and  VARIABLE  WINDS. 

1.  Regular  winds. — Regular  winds  are  those  which  blow 
throughout  the  year  in  the  same  direction.     They  occur  in 
the  neighborhood  of  the   equator,  extending  on   each  side 
about  30  degrees.     From  their  advantage  to  commerce  they 
are  called  trade  winds.     On  the  north  side  of  the  equator 
they  blow  from  the  north-east,  on  the  south  side  they  blow 
from  the  south-east. 

The  trade  winds  arise  from  currents  of  air  flowing  from 
the  polar  regions  towards  the  equator  ;  the  velocity  of  the 
earth  about  its  axis  being  greater  as  WTC  approach  the 
equator,  these  winds  lag  behind  as  it  were,  and  become  in 
clined  to  the  westward,  giving  north-east  winds  on  the  north- 
side,  and  south-east  ones  on  the  south  side  of  the  equator. 

2.  Periodic  winds. — Periodic  winds  are  those  which  at 
regular   intervals    of  time  blow   from  opposite    directions. 
Such  are  the  monsoons  that  prevail  in  the  Indian  ocean, 


(?52.)  How  are  winds  divided?  What  are  regular  winds?  Where  do  they 
occur  ?  What  are  they  called  ?  What  is  their  direction  on  the  north  side  of  the 
equator?  On  the  south  side?  Explain  the  causes  of  the  trade  winds?  What  are 
periodic  winds  ? 


254  POPULAR    PHYSICS. 

blowing  one  half  of  the  year  from  north-east  to  south-west, 
and  the  other  half  in  the  opposite  direction.  When  the  sun 
is  on  the  north  of  the  equator,  the  southern  portion  of  the 
Asiatic  continent  is  warmer  than  the  southern  part  of  Africa, 
and  the  winds  blow  from  south-west  to  north-east;  when 
the  sun  is  on  the  south  side  of  the  equator,  the  reverse  r* 
the  case. 

3.  Variable  winds. — Variable  winds  are  those  which  blow 
sometimes  in  one  direction  and  sometimes  in  another,  with- 
out  any  apparent  law  of  change.  The  further  we  recede 
from  the  equatorial  regions,  the  more  variable  are  the  winds 
in  their  character. 

The  Simoon.— The  Sirocco. 

253.  The  SIMOON  is  a  hot  wind  that  blows  from  the 
deserts  of  Africa.  It  is  felt  in  the  northern  and  north 
eastern  parts  of  the  African  continent.  During  its  preva 
lence  the  thermometer  often  rises  to  120°  F.  In  the  desert 
this  wind  becomes  suffocating  from  its  heat  and  dryness. 
Travellers  exposed  to  it  cover  their  faces  with  thick  cloths, 
and  their  camels  turn  their  backs  to  escape  its  injurious 
effects. 

The  SIROCCO  is  a  hot  wind  that  sometimes  is  felt  in  Italy. 
When  it  blows,  people  remain  in  their  houses,  taking  care  to 
close  every  door  and  window.  Some  suppose  this  to  be  a 
continuation  of  the  simoon  from  the  African  desert,  others 
think  that  it  has  its  origin  in  Sicily. 

Velocity   of  Winds. 

254.  The  velocity  of  winds  is  very  variable.  The 
velocity  is  measured  by  instruments  called  anemometers. 

Explain  the  cause  of  the  monsoons.  What  arc  variable  winds?  When  are  they 
most  variable?  (253.)  "What  is  the  Simoon?  Explain.  What  is  the  Sirocco? 
Explain.  (254.)  What  is  an  anemometer  ? 


SOURCES     OF     HEAT    AND    COLD.  255 

These  consist  of  a  species  of  windmill  attached  to  a  train  of 
wheel-work,  by  means  of  which  the  number  of  revolutions 
per  minute  can  be  registered.  From  the  number  of  revolu 
lions  the  velocity  can  be  computed. 

The  velocity  of  the  gentlest  breeze,  or  zephyr,  is  not  more  than 
one  mile  per  hour  :  a  moderate  wind  travels  at  the  rate  of  4i  to  5 
miles  per  hour,  a  brisk  wind  20  miles  per  hour,  a  tempest  40  to  50 
miles  per  hour,  and  a  hurricane  from  90  to  100  miles  per  hour. 


X. — SOURCES   OF   HEAT   AND   COLD. 

Sources   of  Heat. 

255.  The  principal  sources  of  heat,  are :  the  sun,  elec 
tricity,  chemical  combination  and  combustion,  pressure  and 
percussion,  and  friction. 

1.  The  sun. — The  sun  is  the  most  abundant  source  of 
heat.     We  are  ignorant  of  the  cause  of  heat  in  the  sun's 
rays. 

It  has  been  computed  that  the  heat  received  from  the  sun  by  the 
earth  in  a  year  is  sufficient  to  melt  a  layer  of  ice  extending  over  the 
entire  "lobe,  and  100  feet  in  thickness.  Yet  on  account  of  the  great 
distance  of  the  earth  from  the  sun,  and  its  comparatively  small  size, 
it  can  receive  only  the  minutest  portion  of  the  heat  which  the  sun 
radiates  in  all  directions. 

2.  Electricity. — The  subject  of  heat  due  to  electricity  will 
be  treated  of  under  the  head  of  Electricity. 

3.  Chemical  combination   and  combustion.  —  Chemical 
combinations  are  generally  accompanied  by  a  disengagement 
of  heat.     When  they  take  place  slowly,  the  heat  is  inappre- 

Describo  it.  What  are  the  velocities  of  some  of  the  winds?  (285.)  What  are 
the  principal  sources  of  heat?  What  is  the  most  abundant  source?  What  is  the 
amount  of  heat  received  ~by  the  earth  from  the  sun  in  a  year  t  Explain  chemical 
combination  as  a  source  of  heat. 


256  POPULAR    PHYSICS. 

ciable,  but  when  they  take  place  rapidly,  there  is  often 
produced  an  intense  heat,  and  sometimes  a  development  of 
light. 

Combustion  is  one  form  of  chemical  combination.  The  forms  of 
combustion  exhibited  in  our  fire-places  and  our  lamps,  is  a  combina 
tion  of  the  carbon  and  hydrogen  of  the  wood  and  oil  with  the  oxygen 
of  the  air.  The  products  of  such  forms  of  combustion  are  watery 
vapor,  carbonic  acid,  with  gases  and  volatile  products  that  appear 
under  the  form  of  smoke.  Combustion  is  a  decomposition  of  certain 
substances,  accompanied  by  a  composition  of  new  products.  In  this 
change,  no  element  is  lost,  simply  a  change  of  form  takes  place. 

The  flame  produced  in  combustion,  is  a  mixture  of  gaseous  and 
volatile  matters,  heated  red  hot  by  the  heat  disengaged  in  the  process 
of  combustion. 

The  process  of  respiration  is  a  species  of  slow  combustion,  in  which 
the  carbon  and  other  matter  of  the  blood  unites  with  the  oxygen  of 
the  air.  This  species  of  combustion  gives  rise  to  the  heat  of  the 
body  of  men  and  animals.  This  heat  is  called  animal  heat. 

Fermentation  is  a  chemical  process  that  gives  rise  to  heat. 

4.  Pressure  and  percussion. — Whenever  a  body  is  com 
pressed  so  as  to  reduce  its  volume,  heat  is  developed.  The 
greater  the  compression,  the  greater  the  amount  of  heat 
developed.  If  gas  be  suddenly  and  violently  compressed, 
the  heat  generated  is  sufficient  to  set  fire  to  inflammable 
bodies.  This  subject  was  referred  to  in  the  article  on  Com 
pressibility,  in  which  the  instrument  used  for  inflaming 
tinder  is  figured.  (See  Fig.  4.) 

Percussion  is  a  source  of  heat.  If  a  body,  like  a  piece  of 
metal,  for  example,  be  hammered,  it  soon  becomes  hot.  It 
is  percussion  that  causes  the  heat  when  a  flint  is  struck 
against  a  piece  of  steel.  In  this  case  there  is  a  piece  of  the 
steel  detached  and  rendered  red  hot  by  the  collision. 


Explain  the  phenomena  of  combustion.  What  is  flame?  What  is  respiration? 
What  kind  of  heat  comes  from  respiration?  What  is  fermentation?  Explain 
compression  as  a  source  of  heat.  Illustrate.  Explain  percussion  as  a  cause  of 
heat. 


SOURCES  OF  HEAT  AND  COLD.  257 

5.  Friction.—  Friction  is  the  resistance  which  one  body 
offers  to  another  when  they  are  rubbed  together.  This 
resistance  is  accompanied  with  a  great  development  of  heat. 
In  many  cases,  the  friction  is  so  great  that  the  rubbing 
bodies  are  set  on  fire.  In  this  way  many  savage  tribes  pro 
cure  fire.  Pieces  of  ice  when  rubbed  together,  generate 
heat  enough  to  melt  them.  In  machinery,  the  friction  on 
axles  often  sets  them  on  fire,  especially  when  lubrication  has 
been  neglected. 

Sources   of  Cold. 

256.  The  principal  sources  of  cold  are  :  fusion,  vaporiz 
ation,  expansion  of  gases,  and  radiation  of  heat. 

1.  Fusion.— When  a  body  melts,  it  absorbs  heat  from 
the  surrounding  bodies,  which  becomes  latent  in  the  melted 
body. 

2.  Vaporization. — When  a  liquid   passes  to  a  state  of 
vapor,  it  absorbs  heat,  which  becomes  latent  in  the.  vapor. 
Both  of  these  causes  of  cold  have  been  considered  already. 

3.  Expansion  of  gases. — When   a  gas  is  compressed,  it 
gives  out  heat,  and  conversely,  when  it  expands  it  absorbs 
heat.    This  heat,  it  is,  that  acts  to  keep  the  particles  asunder, 
and  the  further  apart  the  particles  are  kept,  the  greater  the 
amount  of  heat  required. 

Heat  is  the  repulsive  force  that  keeps  a  body  in  a  gaseous  state  at 
all.  or  even  in  a  liquid  state. 

If  air  be  compressed  in  a  condenser  and  then  allowed  to  escape 
into  the  atmosphere,  a  slight  cloud  will  be  formed  ;  this  is  due  to  the 
cold  generated  by  the  expanding  air,  which  condenses  the  vapor  in  the 
air.  This  experiment  illustrates  the  manner  in  which  clouds  are 
formed  in  the  upper  regions  of  the  atmosphere. 


Explain  friction  as  a  source  of  heat.  (256.)  What  are  the  principal  sources  of 
oold?  Explain  fusion  as  a  source  of  coH?  Vaporization.  Expansion  of  gases.  Ex* 
plain  the  formation  of  a  cloud  when  compressed  air  expands. 


258  POPULAR    PHYSICS. 

4.  Radiation. — Eadiation  produces  cold  in  the  radiating 
body,  because  radiation  is  simply  giving  off  heat. 

The  earth,  and  all  bodies  on  its  surface,  are  continually  radiating 
heat.  This  is  compensated  during  the  day  by  the  heat  received 
from  the  sun  ;  in  fact,  the  amount  received  is  greater  than  that 
given  off.  But  at  night  the  reverse  holds  true,  and  a  greater  amount 
is  radiated  than  is  received.  This  cooling  of  the  earth's  surface  is, 
as  has  been  stated,  the  cause  of  dew  and  frost. 

It  is  often  said  that  it  freezes  harder  when  the  moon  shines  than 
when  it  is  concealed  by  clouds.  This  is  the  case,  but  the  moon  has 
nothing  to  do  with  the  freezing.  The  true  explanation  of  the  phe 
nomenon  is  this :  When  the  moon  shines,  it  is  generally  cloudless, 
and  the  radiation  goes  on  more  rapidly,  and  of  course  a  greater  degree 
of  cold  is  produced.  On  the  contrary,  when  the  moon  is  obscured,  it 
is  generally  cloudy:  now  the  clouds  are  good  radiators  of  heat,  and 
the  heat  that  they  send  back  to  the  earth  is  nearly  or  quite  enough 
to  compensate  for  that  radiated  from  the  earth  ;  hence  the  process 
of  freezing  is  either  retarded  or  entirely  prevented 

Plants  are  good  radiators,  hence  they  are  more  likely  to  be  affected 
by  frost  than  other  objects.  To  protect  them  from  frost,  we  cover 
them  with  mats,  which  prevent  radiation,  or  rather  radiate  back  the 
heat  that  the  plants  throw  off. 


Explain  radiation  as  a  cause  of  cold.  Illustrate.  What  effect  has  the  moon  on 
freezing  ?  Why  is  it  colder  when  the  moon  shines  than  when  cloudy  ?  Why  are 
plants  likely  to  be  affected  by  frost  t  How  are  they  protected  f 


CHAPTER  YI. 

OPTICS. 
I .  —  GENERAL        PRINCIPLES 

Definition  of  Optics. 

25*7.  OPTICS  is  that  branch  of  Physics  which  treats  of 
the  phenomena  of  light. 

Definition  of  Light. 

258.  LIGHT  is  that  physical  agent  which,  acting  upon 
the  eye,  produces  the  sensation  of  sight. 

Two  Theories   of  Light. 

259.  Two  theories  have  been  advanced  to  account  for 
the  phenomena  of  light :    the  Emission  Theory,  and  the 

Undulatory,  or  Wave  theory. 

According  to  the  emission  theory,  light  consists  of  in 
finitely  small  particles  of  matter,  shot  forth  from  luminous 
bodies  with  immense  velocity,  which,  falling  on  the  retina 
of  the  eye,  produce  the  sensation  of  sight. 

According  to  the  undulatory  theory,  light,  like  heat,  is 
caused  by  the  vibrations  of  the  molecules  of  bodies.  It 
is  transmitted  by  a  highly  elastic  medium  called  ether. 


(257.)  What  is  Optics  ?  (258.)  What  is  Light  ?  (259.)  What  two  theories 
of  light  have  been  advanced  ?  Explain  the  emission  theory.  Explain  the  wave 
theory. 


260  POPULAR   PHYSICS. 

This  medium,  which  also  transmits  radiant  heat,  extends 
through  space,  penetrates  all  bodies,  and  exists  in  the  inter 
vals  between  their  molecules.  The  molecular  vibrations 
of  a  luminous  body  are  imparted  to  the  neighboring  ether, 
and  are  propagated  through  it  by  a  succession  of  spherical 
waves ;  these  waves  falling  on  the  retina  of  the  eye  excite 
the  sensation  of  sight. 

Light  and  radiant  heat  are  very  closely  related  to  each  other;  they 
are  generated  in  the  same  manner  and  are  propagated  through  the 
same  medium,  but  they  differ  from  each  other  in  their  wave  length, 
and  as  a  consequence  in  their  mode  of  action  on  bodies. 

In  sound  the  particles  of  air  vibrate  to  and  fro  in  the  direction  of 
propagation ;  in  light  and  radiant  heat  the  particles  of  ether  vibrate 
to  and  fro  in  a  direction  perpendicular  to  that  of  propagation.  In 
sound  the  vibrations  are  longitudinal,  or  in  the  direction  of  the  rays ; 
in  light  and  radiant  heat  they  are  transversal,  or  perpendicular  to 
the  rays. 

The  idea  of  transversal  vibrations  may  be  illustrated  by  a  rope 
made  fast  at  one  end  and  held  by  the  hand  at  the  other.  If  the  free 
end  be  moved  rapidly  to  and  fro,  at  right  angles  to  the  rope  a  succes 
sion  of  waves  will  run  along  the  rope,  whilst  the  particles  of  the  rope 
simply  vibrate  back  and  forth  in  perpendiculars  to  the  rope.  If  a 
stone  be  dropped  into  a  pool  of  still  water,  a  series  of  waves  will  be 
propagated  outward,  whilst  the  particles  of  water  simply  rise  and 
fall,  their  motion  being  perpendicular  to  the  direction  of  propagation. 

Luminous  Bodies. — Sources  of  Light. 

2GO.  Bodies  that  emit  light  are  said  to  be  luminous ; 
those  that  are  seen  by  light  derived  from  others  are  said  to 
\)Q  illuminated.  Luminous  bodies  generate  light;  illumi 
nated  bodies  reflect  and  diffuse  it.  The  sun  is  a  luminous 
body ;  the  moon  is  illuminated  by  it. 

The  principal  sources  of  light  are  the  sun,  the  stars,  heat, 
chemical  combination,  phosphorescence,  and  electricity. 

How  i?  lijrht  imparted  to  the  ether?  How  propagated  ?  Relation  between  linht 
and  radiant  heat.  Difference,  llhiftratp  the  idea  of  tranwersal  vibrations.  (260.) 
Define  a  luminous  body.  An  illuminated  body.  Illustrate. 


GENERAL    PRINCIPLES    OF    OPTICS  2G1 

The  ultimate  cause  of  the  sun's  light  is  unknown.  The  sun  is 
surrounded  by  a  gaseous  envelope,  called  the  photosphere,  which  ap 
pears  to  be  in  a  state  of  intense  ignition.  The  molecular  vibrations 
of  this  envelope  are  undoubtedly  tuc  immediate  sources  of  solar  light 
and  solar  heat.  The  stars  are  similar  to  the  sun,  but  on  account  of 
their  enormous  distances  from  us  they  send  us  but  a  small  amount 
of  light  and  heat. 

If  a  body  be  heated  its  molecules  are  thrown  into  vibration,  and 
when  its  temperature  reaches  900°  or  1000°  F.,  it  begins  to  be  lumi 
nous  in  the  dark.  Beyond  that  its  brightness  increases  as  its  temper 
ature  rises. 

The  light  developed  by  chemical  combinations  is  mostly  due  to 
the  heat  that  accompanies  t'-cm.  Combustion  is  an  example ;  the 
affinity  between  the  oxygen  of  the  air  and  the  carbon  of  the  fuel 
causes  them  to  rush  together  under  favorable  circumstance.0.,  thus 
generating  heat  and  ultimately  light  itself. 

Phosphorescence  is  the  property  that  some  bodies  have  of  giving 
out  light  under  certain  conditions ;  it  is  often  observed  in  decaying 
animal  and  vegetable  matter  and  in  some  minerals. 

Electricity  is  the  source  of  a  species  of  light  that  rivals  in  intensity 
that  of  the  sun  itself.  It  will  be  treated  of  hereafter. 


Media. — Opaque  and  Transparent  Bodies. 

261.  A  MEDIUM  is  anything  that  transmits  light;  thus, 
free  space,  air,  water,  and  glass,  are  media. 

Media  owe  their  property  of  transmitting  light  to  the  ether  which 
pervades  them.  This  ether  exists  in  the  spaces  between  the  par 
ticles  of  all  bodies,  but  not  always  in  such  a  state  as  to  permit  the 
transmission  of  light. 

A  TRANSPARENT  BODY  is  one  that  permits  light  to 
pass  through  it  freely,  as  glass,  diamonds,  rock-crystal, 
and  water. 

When  bodies  permit  light  to  pass  through  them,  hut  not 
in  such  quantity  as  to  allow  objects  to  be  seen  through  them, 


What  is  Phosphorescence  ?    Illustrate.    What  is  its  cause ?    (261.)  What  is  a 
Medium  ?    Examples.    What  is  a  Transparent  Body  ? 


262  POPULAR     PHYSICS. 

« 

they  are  called  translucent.     Thus,  scraped  horn,  ground 
glass,  oiled  paper,  and  thin  porcelain  are  translucent. 

An  OPAQUE  BODY  is  one  that  does  not  permit  light  to 
pass  through  it.  Thus,  iron,  wood,  and  granite  are  opaque 
bodies. 

Absorption  of  Light. 

262.  No  body  is  perfectly  transparent ;  all  intercept  or 
absorb  more  or  less  light,  but  some  absorb  much  more  than 
others.    If  light  be  transmitted  through  great  thicknesses  of 
media  which  in  thin  layers  are  transparent,  a  quantity  of 
light  is  absorbed,  and  it  often  happens  that  the  transmitted 
light  is  not  of  sufficient  intensity  to  produce  the  sensation 
of  sight. 

The  atmosphere  seems  perfectly  transparent,  but  it  is  a  known 
fact  that  much  of  the  light  of  the  sun  is  absorbed  in  reaching-  the 
earth,  as  is  shown  by  the  greater  brilliancy  of  the  stars  in  the  higher 
regions,  as  on  mountain  tops.  In  the  high  regions  of  the  atmosphere, 
objects  are  more  clearly  seen  than  nearer  the  earth;  indeed  so  great 
is  the  clearness  of  vision  in  these  regions,  that  it  becomes  exceedingly 
difficult  to  judge  of  distances.  Opaque  bodies  absorb  all  of  the  light 
falling  upon  them  which  is  not  reflected. 

The  physical  cause  of  absorption  of  light  by  bodies  is  some 
peculiarity  of  molecular  constitution,  which  breaks  up  and 
neutralizes  the  waves  of  light  that  enter  them. 

Rays   of  Light.-  Pencils.    Beams. 

263.  A  RAY   of  Light  is  a  line  along  which  light  is 
propagated.     It  is  normal  to  the  advancing  wave  front. 
When  the  source  is  very  distant  the  wave  fronts  are  sensi 
bly  plane  and  the  rays  parallel. 

When  the  ether  is  uniformly  distributed   throughout  a  medium, 

A  Translucent  Body  ?  An  Opaque  Body  ?  (  262.)  Explain  the  phenomenon  of 
absorption.  Effect  of  atmospheric  absorption  t  Physical  cause  of  absorption  ? 
(263.)  What  is  a  ray  of  light? 


GENERAL    PRINCIPLES     OF     OPTICS. 


263 


the  waves  of  light  are  concentric  spheres,  and  the  rays  of  light  are 
straight  lines,  because  a  perpendicular  to  one  wave  front  will  be 
perpendicular  to  all  of  the  successive  stages  of  that  front.  Media,  in 
which  the  ether  is  uniformly  distributed,  are,  with  respect  to  light, 
called  homogeneous.  All  other  media  are  called  heterogeneous. 

When  the  waves  of  light  are  not  concentric  spheres,  the  rays  of 
liaht  are  curved.  Such,  for  example,  are  the  rays  of  light  trans- 
mittcd  through  the  atmosphere. 

A  PENCIL  OF  RAYS  is  a  small  group  of  rays  meeting  in 
a  common  point,  such  as  the  rays  proceeding  from  a  candle 
or  a  lamp. 

When  the  rays  proceed  from  a  common  point,  they  are 
are  said  to  be  divergent.  When  they  proceed  toicards  a 
common  point,  they  are  said  to  be  convergent. 

A  BEAM  OF  RAYS  is  a  small  group  of  parallel  rays,  such 
as  enter  a  small  hole  in  a  shutter,  from  a  distant  body,  as 
the  sun. 

Velocity   of  Light. 

264.  It  was  shown  by  ROOMER,  a  Danish  astronomer, 
in  1678,  that  light  occupies  nearly  8-J-  minutes  in  coming 
from  the  sun  to  the  earth,  which  gives  a  velocity  of  186,000 
miles  per  second. 

He  ascertained  the  velocity  of  light  by  a  succession  of 
observations  on  the  eclipses  of  Jupiter's  first  satellite.  In 
Fig.  155,  S  represents  the  sun,  T7,  the  earth,  J,  Jupiter,  and 
6,  Jupiter's  first  satellite.  The  darkened  portion  of  the 
figure  beyond  Jupiter  represents  the  shadow  of  that  planet 
cast  by  the  sun.  It  is  known  by  computation,  that  Jupiter's 
first  satellite  revolves  about  that  planet  once  in  42  hours, 
28  minutes,  and  36  seconds,  and  by  entering  the  shadow  of 
Jupiter,  is  eclipsed  at  each  revolution. 

What  is  the  direction  of  a  ray  in  a  homogeneous  medium  f  What  is  a  homoge 
neous  medium?  A  heterogeneous  medium  f  Direction  of  a  ray  in  such  a  medi 
um  f  What  is  a  Pencil  of  Rays?  Example.  Convergent?  Divergent?  What  is 
a  Beam  of  Rays?  Example.  (264.)  What  is  the  velocity  of  light?  By  whom 
determined  ? 


26-J:  POPULAR     PHYSICS. 

RCEMER  found  that  as  the  earth  moved  from  I7,  its  nearest 
position  to  Jupiter,  towards  t,  its  most  remote  position, 
the  interval  between  the  consecutive  eclipses  of  the  satel 
lite  gradually  grew  longer,  whilst  in  moving  from  t  back 
again  to  2}  these  intervals  grew  shorter.  The  total  retarda 
tion  in  passing  from  T  to  £,  was  found  to  be  nearly  10^ 
minutes,  and  the  total  acceleration  in  the  remaining  half  of 
the  earth's  revolution  was  also  found  to  be  161  minutes. 
This  was  accounted  for  by  the  fact  that  the  earth  was  mov 
ing  away  from  Jupiter  in  the  first  case,  and  therefore  the 


Fig.  15) 

light  had  to  travel  further  and  further  at  each  eclipse  to 
reach  the  observer,  whilst  in  the  second  case,  the  reverse 
happened. 

RCEMER  therefore  inferred  that  it  required  164-  minutes  for 
a  ray  of  light  to  traverse  the  diameter  of  the  earth's  orbit, 
or  8J-  minutes  for  it  to  pass  over  the  radius  of  that  orbit, 
that  is,  over  a  distance  equal  to  that  of  the  earth  from  the 
sun. 

ROZMER'S  deduction  has  been  confirmed  by  observations 
made  on  the  aberration  of  light,  and  also  by  direct  expert 
ment. 


Explain  the  process  of  RCUMKU'S  discovery.     His  deduction.     Has  it  been  con 
firmed? 


GENERAL     PRINCIPLES     OF     OPTICS.  265 

It  is  difficult  to  conceive  a  velocity  so  great  as  186,000  miles  per 
second,  a  speed  that  would  carry  a  ray  of  light  around  the  earth 
eight  times  in  a  single  second  of  time.  Some  idea,  however,  may  be 
had  of  the  velocity  of  light,  from  the  fact  that  it  would  require  more 
than  two  and  a  half  centuries  for  one  of  our  most  rapid  express 
trains  of  cars  to  run  a  distance  over  which  light  passes  in  8£  minutes. 

It  takes  light  more  than  four  hours  to  reach  us  frorn  Neptune,  the 
most  distant  of  the  planets  of  our  system,  and  it  is  capable  of  proof 
that  light  occupies  more  than  three  years  in  coming  to  us  from  the 
nearest  of  the  fixed  stars.  Now,  if  astronomers  are  right  in  the 
inference  that  the  remotest  stars  visible  in  our  telescopes  are  more 
than  a  thousand  times  as  distant  as  the  nearest  ones,  then  indeed 
must  the  light  that  makes  us  aware  of  their  existence,  have  set  out 
on  its  journey  long  centuries  before  the  beginning  of  the  Christian 
era.  These  conclusions  serve  to  show  the  vastness  of  the  material 
universe,  and  the  comparative  littleness  of  our  own  planet. 


Intensity  of  Light.  —  Photometry. 

265.  The  INTENSITY  OF  LIGHT  is  the  amount  of  disturb 
ance  that  it  imparts  to  the  ether.  It  can  be  shown 
mathematically,  for  light  coming  from  the  same  sources, 
that  the  intensity  varies  inversely  as  the  square  of  the  dis 
tance  from  its  source. 

Hence  we  see  that  light  follows  the  same  law,  with  regard 
to  its  intensity,  that  is  observed  for  gravity  and  sound.  The 
law  of  variation  of  intensity  can  be  verified,  experimentally, 
by  means  of  an  instrument  called  a  photometer. 

A  PHOTOMETER  is  an  instrument  for  comparing  the  inten 
sities  of  different  lights. 

Several  different  instruments  have  been  devised  for  this 
purpose,  one  of  the  simplest  being  that  shown  in  Fig.  156. 

It  consists  of  a  vertical  screen  of  ground-  glass,  -4,  and  a 
vertical  solid  rod,  _Z?,  situated  a  short  distance  in  front  of  it. 

Give  some  illustrations  of  the  immense  velocity  of  light.  (265.)  "What  is  the 
Intensity  of  Light  ?  How  does  it  vary  with  the  distance  ?  What  is  a  Photometer  ? 
Explain  the  one  shown  in  Fig.  156. 

12 


POPULAR     VHYSICS. 


If  two  equal  lights  are  placed  at  equal  distances  from  .Z?,  it 
is  found  that  the  shadows  which  _Z?  casts  upon  A,  are  of  the 
same  tint.  If  one  light  be  placed  at  any  distance,  and  four 


Fig.  15G. 

equal  lights  be  placed  at  twice  the  distance,  the  shadows 
will  be  of  the  same  tint ;  this  is  the  case  shown  in  the  figure. 
It  will  require  nine  equal  lights  at  three  times  the  distance, 
sixteen  at  four  times  the  distance,  and  so  on,  to  produce  the 
same  effect.  This  experiment  confirms  the  law  of  variation 
of  intensity  according  to  the  inverse  square  of  the  dis 
tance. 

i  To  use  the  photometer  to  compare  the  intensities  of  any 
two  lights,  let  them  be  placed,  by  trial,  at  such  distances 
from  .5,  that  the  shadows  cast  on  A  are  of  exactly  the  same 
tint ;  then  will  their  intensities  be  to  each  other  inversely  as 
the  squares  of  their  distances  from  the  rod,  B. 

How  is  the  photometer  used? 


REFLECTION    OF    LIGHT.  267 

II.  —  REFLECTION     OF     LIGHT.  —  M  »R  R  O  R  S  . 

Reflection  of  Light. 

266.  When  light  passes  obliquely  from  one  medium  to 
another,  it  is  separated  into  two  parts,  one  of  which  is 
driven  back  and  remains  in  the  first  medium,  whilst  the 
other  passes  on  and  enters  the  second  medium.  The  part 
that  is  driven  back  is  said  to  be  reflected,  and  the  deviating 
surface  is  called  a  reflector. 

Reflection  of  light  is  explained  in  the  same  way  as  reflection  of 
sound.  In  case  of  light  the  wave  lengths  are  so  small  that  the  most 
highly  polished  surfaces  are  comparatively  rough.  Hence,  only  a 
part  of  the  reflected  light  appears  to  follow  the  regular  laws  ;  the 
rest  is  irregularly  reflected  or  diffused.  The  amount  of  light  reflected, 
as  well  as  the  relation  between  that  which  is  regularly  and  that 
which  is  irregularly  reflected,  depends  on  the  obliquity  of  incidence, 
the  nature  of  the  second  medium,  and  the  polish  of  the  deviating 
surface, 

Light  that  is  irregularly  reflected  enables  us  to  see  objects;  thus, 
the  light  falling  on  a  sheet  of  paper  is  scattered  or  diffused  so  as  to 
render  it  visible  in  all  directions.  If  a  reflector  were  perfectly  smooth 
it  would  be  invisible ;  we  should  simply  see  in  it  the  images  of  other 
objects. 

It  is  the  diffused  light  reflected  by  the  clouds,  the  air,  the  earth, 
and  objects  upon  it,  that  illuminates  our  rooms  and  renders  objects 
visible  which  do  not  receive  the  direct  rays  of  the  sun. 

If  we  look  out  from  our  houses  we  see  objects  clearly  by  means  of 
this  diffuse  light,  because  they  receive  much  light,  and  therefore 
reflect  much;  but  if  we  look  from  without  into  a  house,  we  see 
objects  with  less  distinctness,  because  they  receive  but  little  light, 
and  therefore  they  reflect  but  little. 

It  is  now  proposed  to  explain  the  laws  of  regular  reflection. 


(266.)  What  is  reflection  of  light  ?  What  is  a  reflector  ?  How  is  reflection  of 
light  explained  ?  What  is  diffused  light  ?  How  are  we  able  to  see  non-luminous 
bodies? 


208 


POPULAR     PHYSICS. 


Definitions   of  Terms, 

267.     The  ray  that  falls  upon  a  reflecting  surface  is  called 
the  incident  ray  ;  thus,  CD,  Fig.  157,  is  an  incident  ray. 


Fig.  157. 

The  point  where  the  incident  ray  meets  the  reflecting 
surface,  is  called  the  point  of  incidence  ;  thus,  D  is  a  point 
of  incidence. 

The  angle  that  the  incident  ray  makes  with  the  normal 
to  the  reflecting  surface  at  the  point  of  incidence,  is  called 
the  angle  of  incidence ;  thus,  CD  A  is  an  angle  of  in 
cidence. 


(267.)  What  is  an  incident  ray?    Example.    The  point  of  incidence  ?    Example. 
The  angle  of  incidence  ?    Example. 


REFLECTION     OF     LIGHT.  269 

The  plane  that  passes  through  the  incident  ray  and  the 
normal  is  called  the  plane  of  incidence  ;  thus,  the  plane 
through  CD  and  DA,  is  a  plane  of  incidence. 

The  ray  driven  off  from  the  reflecting  surface  is  called 
the  reflected  ray  ;  thus,  DB  is  a  reflected  ray. 

The  angle  that  the  reflected  ray  makes  with  the  normal 
h  called  the  angle  of  reflection  ;  thus,  BDA  is  an  angle  of 
reflection. 

The  plane  of  the  reflected  ray  and  the  normal  is  called 
the  plane  of  reflection  ;  thus,  the  plane  of  BD  and  DA  is  a 
plane  of  reflection. 

Laws  of  Reflection. 

26§.  The  following  laws  are  shown  by  theory,  and  con 
firmed  by  experiment : 

1.  The  planes  of  incidence  and  reflection  coincide  /  both 
are  normal  to  the  reflecting  surface  at  the  point  of  incidence. 

2.  The  angles  of  incidence  and  reflection  are  equal  •  this 
is  true  whatever  may  be  the  angle  of  incidence. 

Direction  in   which  objects  are   seen. 

269.  Whenever  the  rays  of  light  proceed  directly  from 
an  object  to  the  eye,  we  see  the  body  exactly  where  it  is. 
When  by  reflection,  or  any  other  cause,  the  rays  are  bent 
from  their  primitive  direction,  we  no  longer  see  bodies  in 
their  proper  position.  They  appear  to  be  in  the  direction 
from  which  the  ray  enters  the  eye. 

This  is  illustrated  in  Fig.  158.  A,  represents  a  body 
from  which  a  ray  of  light,  proceeding  in  the  direction  AB, 
is  deviated  or  bent  at  J5,  so  as  to  assume  the  new  direction, 
BC.  The  eye  receives  the  ray  from  the  direction  BC, 

The  plane  of  incidence?  Example.  The  reflected  ray  ?  Example.  The  angle  of 
reflection?  Example.  The  plane  of  reflection?  Example.  (268.)  What  is  the 
first  law  of  reflection?  What  is  the  second  law  of  reflection?  (269.)  In  what 
direction  do  objects  appear  to  the  eye  ?  Illustrate. 


270  POPULAR     PHYSICS. 

and  in  consequence  the  object,  A,  appears  to  be  situated  at 


Fig.  158. 

some  point,  a.     This  principle  is  of  importance  in  explaining 
certain  phenomena  produced  by  reflectors  and  lenses. 

Mirrors. 

270.  A  MIRROR  is  a  body  with  a  polished  surface,  em 
ployed  to  form  images  of  objects. 

The  best  reflecting  surfaces  are  those  of  polished  metals. 
Our  ordinary  looking-glasses  are  composed  of  plates  of 
smooth  glass,  upon  the  back  of  which  is  fastened  a  thin  layer 
of  tin  and  quicksilver. 

This  mixture,  called  an  amalgam,  offers  an  excellent  re 
flecting  surface,  and  it  is  from  it  that  the  principal  reflection 
takes  place.  The  glass  serves  to  give  the  proper  smooth 
ness  to  the  amalgam,  as  well  as  to  protect  it  from  injury 
a:xl  tarnish.  There  is,  however,  a  reflection  from  the  oujter 
surface  of  the  glass,  giving  rise  to  feeble  images,  which 
render  such  reflectors  objectionable  for  optical  purposes. 
Hence  it  is,  that  reflectors  for  telescopes,  and  the  like,  are 
generally  made  of  alloys,  or  mixtures  of  hard  metals,  which 
admit  of  a  high  polish.  Such  a  mirror  is  called  a  speculum. 

Mirrors  are  of  two  kinds,  plane  and  curved. 

Plane  Mirrors. 

271.  A  PLANE  MIRROR  is  pne  in  which  the  reflecting 
surface  is  plane. 

(2 7O.)  What  is  a  Mirror?  What  are  the  best  reflecting  surfaces?  What  are 
looking-glasses?  Explain  their  construction?  What  is  a  speculum?  How  many 
kinds  of  mirrors  are  there  ?  What  are  they  ?  (271.)  What  is  a  Plane  Mirror? 


MIRRORS. 


271 


We  have  an  example  of  plane  mirrors  in  the  ordinary 
looking-glasses  of  our  houses.  The  surface  of  still  water? 
which  reflects  surrounding  objects,  and  the  surface  of  quick 
silver,  when  at  rest,  are  additional  examples.  The  latter  is 
often  used  with  the  sextant  in  measuring  the  altitudes  of 
the  stars;  it  is  also  used  in  adjusting  astronomical  instrr. 
inents. 

Images  formed  by  Plane  Reflectors. 

272.  An  IMAGE  of  an  object  is  a  picture  or  representa 
tion  of  that  object,  formed  by  a  reflector,  or  by  a  lens. 


The  manner  of  forming   images   by  plane   reflectors  is 
illustrated  in   Fig.  159.     A  pencil   of  rays   coming  from  a 

Give  examples.     (272  )  What  is  an  Image  of  an  object?     Explain  the  manner  of 
forminj  the  image  of  a  point. 


272 


POP UL All    PHYSICS. 


point,  is  reflected  so  as  to  reach,  the  eye.  Because  the 
angles  of  incidence  and  reflection  are  equal  (Art.  268),  each 
ray  will  have  the  same  inclination  to  the  mirror  after  reflec 
tion  that  it  had  before  incidence.  Hence  the  reflected  rays, 
on  being  produced  back,  will  meet  at  a  point  as  far  behind 
the  reflector  as  the  point  of  the  object  is  in  front  of  it. 
Now,  because  the  eye  sees  objects  in  the  direction  from 
which  the  rays  reach  it  (Art.  269),  the  point  appears  to  be 
as  far  behind  the  mirror  as  it  really  is  in  front  of  it.  The 
representation  of  the  point  thus  formed,  is  its  image. 

What  has  been  said  of  a  single  point  is  true  of  all  points. 
Hence,  if  we  suppose  pencils  of  rays  to  proceed  from  every 
point  of  an  object,  as  shown  in  Fig.  160,  each  point  will 


Fig.  160. 


Explain  the  manner  of  forming  the  image  of  an  object. 


M1KKOKS.  273 

have  its  own  image  as  far  behind  the  mirror  as  the  point  is 
in  front  of  it.  The  assemblage  of  images  thus  formed 
makes  up  the  image  of  the  object. 

Nature  of  the  Images  formed. 

273.  It  will  be  seen  from  an  inspection  of  Fig.  160,  that 
the  image  of  the  child's  right  hand  is  on  the  left  of  the 
image  in  the  glass,  and  that  the  image  of  the  child's  left 
foot  is  on  the  right  of  the  image  in  the  glass,  that  is,  the 
image  is  reversed  laterally.     This  comes  from  the  fact,  that 
the  image  of  each  point  is  as  far  behind  the  mirror  as  the 
point  is  in  front.     Hence  we  say,  that  an  object  and  its 
image  arc  symmetrically  situated  with  respect  to  the  mirror. 

We  see  also  from  what  has  been  said,  that  the  image  is 
erect,  and  equal  in  size  with  the  object. 

The  rays  that  reach  the  eye  appear  to  come  from  an 
image  which  does  not  in  reality  exist.  The  image  is  only 
apparent.  Such  images  are  called  virtual. 

A  VIRTUAL  IMAGE  is  an  image  that  appears  to  exist, 
and  which  would  be  found  by  producing  the  deviated 
pencils  of  rays  backward,  till  they  meet  in  points. 

Multiple  Images  from   Looking-glasses. 

274.  Metallic  mirrors,  or  specula,  as  they  are  called,  having  but 
one  reflecting  surface,  form  but  a  single  image.     Glass  mirrors  have 
two  reflecting  surfaces,  the  front  surface  of  the  glass,  and   the  me 
tallic  surface  at  the  back  of  the  glass.     An  image  is  formed  by  each 
of  these  surfaces,  but  that  formed  by  the  latter  is  the  more  striking, 
because  the  first  surface  reflects  only  a  small  portion  of  the  light. 

This  formation  of  two  images  by  glass  mirrors  renders  them  unfit 
for  many  optical  purposes.  The  double  image,  formed  by  placing  a 
point  against  the  glass,  enables  us  to  judge  of  the  thickness  of  the 
glass. 

(273.)  How  are  the  object  and  its  image  by  a  plane  reflector  situated?  Is  tho 
image  real  or  apparent ?  Why?  What  is  a  Virtual  T mage?  (274.)  Why  do  glass 
mirrors  form  two  images  ?  What  is  the  objection  to  this  duplication  ?  ilo\o  do  wf 
'nidge  of  the  thickness  of  glass  ? 

12* 


2T4 


POPULAR     PHYSICS. 


Reflection  by  Transparent  Bodies. 

We  have  just  seen  that  glass,  notwithstanding  its  trans 
parency,  reflects  light  enough  to  form  an  image.  The  same  is  the 
case  with  other  transparent  bodies,  of  which  water  forms  a  con- 
spicuous  example. 


Fig.  161  represents  the  phenomenon  of  reflection  from  the  surface 
of  still  water.  It  shows  how  the  reflected  ray^  produce  images  of 
objects  above  the  water,  which  are  symmetrically  disposed  with 
respect  to  the  surface  of  the  water.  The  case  is  entirely  the  same 
as  though  the  images  had  been  formed  by  a  horizontal  looking-glass. 

Curved  Mirrors. 

276.  A  CURVED  MIRROR  is  one  in  which  the  reflecting 
surface  is  curved.  The  most  important  class  of  curved 
mirrors,  is  that  in  which  the  reflecting  surface  is  a  portion 


(275.)  Do  transparent  bodies  reflect  light  ?    Explain  the  reflection  from  water. 
(276.)  What  is  a  Curved  Mirror  ? 


MIRRORS.  275 

of  a  sphere.  When  the  reflection  takes  place  from  the 
hollow  or  concave  side,  the  mirror  is  called  concave  /  when 
the  reflection  takes  place  from  the  outer  or  convex  side,  the 
mirror  is  called  convex, 

Concave  Mirrors. 

aw.  A  CONCAVE  MIRROR  is  one  in  which  the  reflection 
takes  place  from  the  concave  side  of  a  curved  surface. 

We  shall  consider  the  case  in  which  the  reflecting  surface 
is  a  segment  of  a  sphere. 

The  following  definitions  apply  equally  to  concave  and 
convex  mirrors : 

The  middle  point  of  the  mirror  is  called  its  vertex.  The 
centre  of  the  sphere,  of  which  the  mirror  forms  a  part,  is 
called  the  optical  centre.  The  indefinite  straight  line 
through  the  optical  centre  and  the  vertex,  is  called  the 
principal  axis,  or  sometimes  simply  the  axis.  Any  plane 
section  through  the  axis  is  called  a  principal  section. 

Thus,  JfiV,  Fig.  162,  represents  a  principal  section  of  a 
concave  mirror,  A  is  its  vertex,  C  its  optical  centre,  and 
A X  is  its  principal  axis. 


Fig.  162. 

It  is  to  be  observed,  that  in  practice  the  surface  of  a  curved  mirror 
is  only  a  very  small  part  of  the  surface  of  the  sphere  of  which  it, 
forms  a  part. 

Concave?  Convex?  ( 277-)  What  is  a  Concave  Mirror  ?  What  is  the  vertex? 
The  optical  centre?  The  principal  axis?  A  principal  section?  Illustrate. 


276 


POPULAR    PHYSICS. 


Principal  Focus   of  a   Concave   Mirror. 

A  Focus  is  a  point  in  which  deviated  rays  meet. 
If  the  incident  rays  are  parallel  to  the  axis,  the  focus  is  call 
ed  the  Principal  Focus. 

In  Fig.  162,  £Tand  si,  are  two  rays  parallel  to  the  axis, 
CI  and  Ci  are  normals  at  the  points  of  incidence,  I  and  L 
IF  and  iF  are  reflected  rays,  making  the  angles  of  reflec 
tion  equal  to  the  angles  of  incidence.  When  the  mirror  is 
small,  compared  with  the  whole  sphere,  all  the  rays  parallel 
to  the  axis  are  reflected  to  the  same  point,  F.  Hence,  from 
the  definition,  F  is  the  principal  focus.  It  can  be  shown 
that  the  principal  focus  is  on  the  axis,  and  midway  between 
the  vertex  and  optical  centre.  We  shall  always  designate 
the  principal  focus  by  the  letter  F. 


(278.)  What  is  a  Focns  ?    The  Principal  Focus?     Illustrate. 


MIKEOKS.  277 

Fig.  163  shows  the  manner  or  determining  the  principal  focus  by 
experiment,  making  use  of  a  beam  of  light  coming  from  the  sun. 
In  this  form  the  concave  reflector  may  be  used  to  collect  the  rays  for 
the  purpose  of  developing  a  great  amount  of  heat. 


Conjugate  Foci. 

279.  If  the  rays  of  light  emanate  from  some  point  of 
the  axis  not  infinitely  distant  from  the  mirror,  they  will  be 
"brought  to  a  focus  at  some  point  of  the  axis,  generally 


Fig.  164. 

different  from  _F.  Thus,  in  Fig.  164,  the  pencil  of  rays, 
coming  from  the  point  B,  are  brought  to  a  focus  at  ft,  be 
tween  F  and  C.  Had  the  rays  emanated  from  ft,  they 
would  have  been  brought  to  a  focus  at  J5.  These  points 
are  so  related  as  to  receive  the  name  of  conjugate  foci. 
Hence  we  have  the  following  definition  : 

CONJUGATE  Foci  are  any  twro  points  so  related  that  a 
pencil  of  light,  emanating  from  either  one,  is  brought  to  a 
focus  at  the  other. 

Thai?  one  from  which  the  light  actually  proceeds  is  called 
the  radiant ;  thus,  in  Fig.  164,  B  is  the  radiant. 


Explain    the    manner   of  determining  the,   principal  focus   by    experiment. 

(  279.)  What  arc  Conjugate  Foci  ?    The  radiant  ? 


278  POPULAR   PHYSICS. 

The  following  arc  sonic  properties  of  conjugate  foci  of 
concave  mirrors  : 

If  the  radiant  is  on  the  axis  and  at  an  infinite  distance 
from  the  mirror,  the  rays  will  be  parallel,  and  the  corre 
sponding  focus  is  at  F,  (Fig,  1G2). 

As  the  radiant  approaches  the  mirror,  the  focus  recedes 
from  it. 

If  the  radiant  is  beyond  the  optical  centre,  C,  the  focus 
is  between  If7  and  C. 

If  the  radiant  is  at  (7,  the  focus  is  at  C  also. 

If  the  radiant  is  between  C  and  F,  the  focus  is  beyond  (7, 
in  the  direction  CX. 

If  the  radiant  is  at  JF]  the  focus  is  at  an  infinite  distance, 
that  is,  the  reflected  rays  are  parallel. 


Fig.  165. 

If  the  radiant  is  between  F  and  A^  as  shown  in  Fig.  165, 
the  rays  are  reflected  so  as  to  diverge,  and  on  being  pro 
duced  backwards,  meet  at  p.  In  this  case  the  focus  is 
behind  the  mirror,  and  is  said  to  be  virtual  (Art.  273). 

If  the  radiant  is  not  on  the  axis,  the  pencil  of  rays  is  ob 
lique,  but  it  is  still  brought  to  a  focus,  and  if  not  far  distant 
from  the  axis,  the  radiant  and  focus  enjoy  properties  entirely 
analogous  to  those  just  explained. 

If  the  radiant  is  at  an  infinite  distance,  where  is  the  conjugate  focus?  If  the  radi 
ant  approaches  the  mirror,  how  does  the  focus  move  ?  Where  do  they  meet?  If  the 
radiant  is  at  the  principal  focus,  where  is  the  conjugate  focus?  When  is  the  focus 
virtual?  Explain  the  law  of  an  oblique  pencil  of  rays. 


MIRRORS. 


279 


Formation  of  Images  by  Concave  Reflectors. 

280.  If  an  object  be  placed  in  front  of  a  concave  mir 
ror,  a  pencil  of  rays  will  proceed  from  each  point  of  the 
object,  which  after  reflection  will  be  brought  to  a  focus 
either  real  or  virtual.     The  collection  of  foci  thus  formed, 
make  up  the  image  of  the  object. 

Real   Images. 

281.  If  the  object  is  further  from  the  mirror  than  the 
principal  focus,  the  image  will  be  inverted,  and  real. 


••'r:!':""'.;-,. 


Fig.  1GG.  • 

Fig.  166  represents  an  inverted  image  formed  by  a  con 
cave  reflector.  That  the  image  is  real,  may  be  shown  by 
throwino-  it  on  a  screen. 

O 

(  28O.)  How  is  an  image  of  an  object  formed?    (281.)  When  is  the  image  real 
and  inverted? 


280 


POPULAR     PHYSICS. 


Fig.  167  shows  the  course  of  the  rays  in  forming  a  real 
image  by  means  of  a  concave  mirror.  In  this  case  the  image 
of  a  distant  church  is  formed  and  thrown  upon  a  sheet  of 
paper ;  the  image  is  a  perfect  picture,  not  only  in  outline 
but  in  color ;  its  only  defect  is  that  it  is  inverted. 


Fig   107. 


When  the  object  is  at  a  greater  distance  from  the  mirror 
than  the  optical  centre,  the  image  is  less  than  the  object ; 
when  the  object  is  between  the  optical  centre  and  the  prin 
cipal  focus,  the  image  is  greater  than  the  object.  In  this 
case  the  reflector  may  be  used  as  a  magnifier. 

Explain  the  course  of  the  rays  in  forming  an  image.    When  is  the  image  smaller 
than  the  object  ?    When  larger  ? 


MIRRORS. 


281 


Virtual  Images. 

282.  When  the  object 
is  between  the  principal  fo 
cus  and  the  mirror,  the  im- 
ao-e  is  virtual  and  erect,  as 
shown  in  Fig.  168.  Further 
more,  it  is  larger  than  the 
object,  or  magnified. 

Fio-.  169  shows  the  course 

^ 

of  the  rays  in  forming  a  vir 
tual  and  erect  image.  The 
face  is  between  the  principal 
focus,  F,  and  the  mirror. 
The  pencils  of  rays  from  a 
and  b  are  reflected  so  as  to 

diverge  from  the  virtual  foci,  A  and  B.     It  is  easily  seen 
that  the  image  is  larger  than  the  object. 


Fig.  169. 


(282.)   When  is  the  image  virtual?    Explain  the  course  of  the  rays  in  forrr.i 
virtual  image. 


282 


POPULAR     PHYSICS. 


Formation  of  Images  by  Convex  Reflectors. 

283.  In  convex  mirrors  the  reflection  takes  place  from 
the  outer  or  convex  surface. 

From  what  has  been  said  of  concave  mirrors,  it  will  readily 
be  seen  how  images  are  formed  by  convex  mirrors.  The 


Fig.  170. 


images  formed  in  this  case  are  always  virtual,  always  erect, 
and  always  smaller  than  the  object,  as  is  shown  in  Fig.  170. 


Fig.  171. 

Fig.  171  shows  the  course  of  the  rays  in  the  formation  of 

(  283  )  Do  convex  reflectors  form  erect  or  inver'ed  images?    Are  they  magnified 
or  diminished  ?    Explain  the  course  of  the  rays  in  this  reflector. 


KEFIl  ACTION     OF    LIGHT. 


283 


an  image  by  means  of  a  convex  mirror.  After  what  has 
been  said  in  the  preceding  article,  this  figure  needs  no 
explanation. 

III.  —  REFRACTION   OF   LIGHT.  —  LENSES. 

Refraction. 

284.  If  a  beam  of  light  fall  obliquely  on  a  surface  that 
separates  two  media,  it  is  divided  into  two  parts ;  one  is 
reflected  and  remains  in  the  first  medium,  the  other  enters 
the  second  medium,  and  is  partially  absorbed  and  partially 
transmitted.    The  transmitted  rays  change  direction  at  the 
point  of  incidence.     This  change  of  direction   is  called 
refraction.      Its   amount   depends   on   the   nature  of  the 
media,  and  also  on  the  obliquity  of  incidence. 

The  cause  of  this  change  of  di 
rection  is  a  change  in  the  elasticity 
and  density  of  the  ether  in  passing 
from  one  medium  into  the  other, 
which  causes  a  change  in  the  ve 
locity  of  the  ray.  Thus  the  dens 
ity  and  elasticity  of  ether  in  wa 
ter  are  different  from  what  they 
are  in  the  atmosphere,  so  that  light 
travels  considerably  faster  in  the 

latter  medium  than  in  the  former.  This  causes  a  ray,  on  passing 
from  air  into  water  to  bend  towards  the  normal  at  the  point  of 
incidence,  as  shown  in  Fig.  172.  Thus,  LA,  is  bent  from  its 
course  so  as  to  take  the  direction  AK.  In  passing  from  water  to 
air,  the  ray  is  bent  away  from  the  normal,  just  the  reverse  of  wlr  b 
happens  when  light  passes  from  air  into  water. 

Definitions. 

285.  The  ray  before  refraction  is   called  the  incident 
ray  ;  thus,  LA  (Fig.  172),  is  an  incident  ray. 


(284.)  What  is  Refraction?     What  in  the  cause  of  refraction?     Which  way  ia 
the  ray  bent  ?    (285.)  What  is  the  incident  ray  ?    Illustrate. 


284  POPULAR    PHYSICS. 

The  point  at  which  the  ray  is  deviated  or  bent,  is  called 
the  point  of  incidence  ;  thus,  A  is  a  point  of  incidence. 

The  ray  after  deviation  is  called  the  refracted  ray  ;  thus, 
AK\s  a  refracted  ray. 

The  angle  that  the  incident  ray  makes  with  the  normal 
at  the  point  of  incidence  is  called  the  angle  of  incidence, 
and  the  plane  of  this  angle  is  the  plane  of  incidence.  Thus, 
LAB  is  an  angle  of  incidence,  and  the  plane,  LAB,  is  the 
plane  of  incidence. 

The  angle  that  the  refracted  ray  makes  with  the  normal 
at  the  point  of  incidence  is  called  the  angle  of  refraction,  and 
the  plane  of  this  angle  is  the  plane  of  refraction  ;  thus,  the 
angle  KA  C  is  an  angle  of  refraction,  and  the  plane  of  this 
angle  is  a  plane  of  refraction. 

Laws  of  Refraction. 

28O.  When  light  passes  from  any  given  medium  into 
another,  no  matter  what  may  be  the  angle  of  incidence,  it 
always  conforms  to  the  following  laws  : 

1.  The  planes  of  incidence  and  refraction  coincide,  both 
being  normal  to  the  surface  separating  the  media,  at  the 
point  of  incidence. 

2.  The  sine  of  the  angle  of  incidence  is  equal  to  the  sine 
of  the  angle  of  refraction  multiplied  by  a  constant  quan 
tity. 

This  constant  multiplier  is  called  the  index  of  refraction, 
and  is  equal  to  the  ratio  of  the  velocities  of  light  in  the 
two  media. 


What  is  the  point  of  incidence?  Illustrate.  The  refracted  ray?  Illustrate. 
The  angle  and  plane  of  incidence  ?  Illustrate.  The  angle  and  plane  of  refraction  ? 
Illustrate.  (286.)  What  is  the  first  law  of  refraction?  The  second  law  ?  What 
is  the  index  of  refraction  ? 


REFRACTION     OF    LIGHT.  285 

The  second  law  may  be  illustrated  by  the  figure  in  the  margin. 
Let  A  be  the  point  of  incidence  on  a  surface   separating  air  from 
vvaier.      With  A  as  a  centre,  describe  a  circle,  BmCp.     Let  LA  be 
an  incident  ray,  and  AK  the  refracted 
ray.     Draw  mn  and  pq  perpendicular 
to  the  normal.  BC.     Then  will  these 
lines  be  the  sines  of  the  angles  of  inci 
dence  and  refraction,  and  we  shall  have 
in  the  particular  case  of  air  and  water, 
mn  equal  to  pq  multiplied  by  1|,  what 
ever    may  be   the  inclination  of   LA. 
Here  H  is  the  index  of  refraction.    For 
air  and   glass  the  index  of  refraction 


Refractive  power  of  Bodies. 

287.  Different  bodies  possess  different  refractive  powers.    NEW 
TON   observed  that,  as'  a  general    rule,  the   refractive    power  was 
greatest  for  combustible  bodies,  or  bodies  containing  combustible 
elements,  such  as  alcohol,  ether,  oils,  &c.,  which  contain  both  hydro 
gen  and  carbon.      He  found  that  the  diamond   was    more    highly 
refractive   than  any  other  body,  and  hence  inferred  that  it  was  a 
combustible  body,  an  inference  that  has  since  been  confirmed.     It  is 
to  its  high  refractive  power  that  the  diamond  owes  its  brilliancy  as 
a  jewel.     Gases   are  not  so  highly  refractive  as  liquids,  but  their 
refractive  power  may  be  increased  by  compression,  which  augments 
their  density. 

Experimental  proofs   of  Refraction. 

288.  If  a  beam  of  light  be  introduced  through  a  hole  in 
a  shutter  of  a  dark  room,  and  allowed  to  fall  upon  the  sur 
face  of  water  in  a  glass  vessel,  as  shown  in  Fig.  173,  the 
bending  of  the  beam  as  it  enters  the  water  may  be  seen  by 


How  may  the  second  law  l>e,  illustrated  ?  (  287.)  Do  all  bodies  refract  equally  f 
Explain  NEWTON'S  views  ?  (288)  Explain  the  method  of  proving  refraction  ex- 
perimen  tatty? 


286 


POPULAR     PHYSICS. 


the  eye.  The  course  of  a  ray  in  the  air  may  be  rendered 
more  apparent  by  filling  the  air  with  fine  dust  or  smoke,  as, 
for  example,  the  smoke  from  gunpowder. 


Fijr.  173. 

Let  a  piece  of  money  be  placed  at  the  bottom  of  an  empty 
vessel,  and  then  take  a  position  such  that  the  coin  shall  just 
be  hidden  by  the  side  of  the  vessel.  Whilst  in  this  position, 
if  water  be  poured  into  the  vessel,  the  rays  from  the  coin 
will  be  refracted  so  as  to  render  it  visible.  The  effect  of 
refraction  in  this  and  similar  cases,  is  to  make  the  bottom  of 
the  vessel  appear  higher  than  it  is  in  reality,  as  shown  in 
Fig.  174. 

Explain  a  second  methol  ? 


REFRACTION     OF    LIGHT. 


287 


.Vis.  174. 


Some  effects  of  Refraction. 

289.  One  of  the  effects  of  refraction  was  explained  in  the  last 
article.  The  principle  has  numerous  applications.  To  a  person 
standing  on  the  shore,  a  fish  in  the  water  appears  higher  than  his 


Fig.  175. 
real  position,  as  is  shown  in  Fig.  175. 


(289.)  Why  does  a  fish  seem  higher  in  the  water  than  he  really  is  f 


288 


POPULAR    PHYSICS. 


If  a  stick  be  partially  plunged  into  water,  the  portion  immersed 
will  be  thrown  up  by  refraction,  and  the  stick  will  appear  bent,  as 
shown  in  Fig.  176. 


Fig.  1TG. 

Refraction  has  the  effect  to  make  the  heavenly  bodies  appear 
higher  than  they  are.  and  thereby  causes  them  to  rise  earlier  and  set 
later  than  they  would  do  weje  there  no  atmosphere. 

Total  Reflection. 

290.  If  light  fall  on  a  surface  that  separates  a  medium 
from  one  that  is  less  refractive,  there  is  a  limit  beyond 
which  it  will  not  pass  from  the 
first  medium  into  the  second, 
at  that  limit  light  is  totally 
reflected. 

Let  BMC  be  a  glass  globe  half  full 
of  water.  The  ray,  LA,  being  normal 
to  the  globe,  is  not  refracted  in  enter 
ing,  but  if  the  angle,  GAL,  be  small 
enough,  it  is  refracted  at  A,  taking 
same  direction  AR.  If  the  angle  of 
incidence  exceed  41°,  the  ray  can  no 
longer  pass  through  the  surface  AM, 
but  is  totally  reflected  and  remains  in  the  first  medium,  taking  same 
direction  Ar. 


Explain  tots}  reflection.    Illustrate. 


REFRACTION    OF    LIGHT. 


289 


Mirage. 

291.  MIRAGE  is  an  atmospheric  phenomenon  depend 
ent  on  extraordinary  refraction  and  total  reflection. 

Sometimes  a  layer  of  atmosphere  next  the  earth  becomes  a  reflector, 
and  in  that  case  portions  of  the  earth  appear  to  the  traveller  like 
lakes  and  ponds  •  such  appearances  are  frequent  in  desert  countries 
when  the  beat  is  intense.  To  heighten  the  illusion,  trees  are  often 
^cen  reflected  from  the  surfaces  of  these  apparent  ponds.  An  exam 
ple  of  this  kind  is  shown  in  Fig.  178.  The  rays  coming  from  the  top 


Fig.  178. 

of  the  tree  on  the  left  of  the  picture,  are  totally  reflected  at  a,  from 
a  layer  of  the  atmosphere,  and  reach  the  eye  of  the  observer  at  Ihe 
tent.  The  observer  refers  the  position  of  the  tree  top  backwards 
along  the  direction  of  the  dotted  line,  which  causes  the  tree  to  appeal- 
inverted.  In  this  case  both  the  tree  and  its  image  are  seen. 

Now  if  we  suppose  both  to  be  thrown  up  by  extraordinary  refrac 
tion,  we  shall  have  a  phenomenon  not  unfrequently  noticed,  in  which 
the  object  is  seen  elevated  in  the  air,  accompanied  by  an  inverted 
image. 

Explain  the  phenomena  of  mirage. 


OPULAil    PHYSICS, 


Double  Refraction.— Polarization, 

Certain  crystalline  substances  have  the  power 
to  separate  a  transmitted  beam  into  two  parts,  so  that  ob 
jects  seen  through  them 
appear  double.  This 
phenomenon,  called 
double  refraction,  de 
pends  on  the  molecular 
arrangement  of  the 
body,  which  causes  the 
contained  ether  to  have 
different  degrees  of  elas 
ticity  in  different  directions. 

Iceland  spar  is  an  example  of  double  refracting  bodies.    Its  crystals 
can  be  reduced  by  cleavage  to  the  form  of  an  equilateral  rhomb,  as 
shown   in   the  figure.     The   particles   arc   symmetrically   arranged 
about  the  shortest  diagonal  ab,  and 
this  is  called  the  axis.    The  contained 
ether  has  its  maximum  density  in  the 
direction  of  the  axis,  and  its  minimum 
deusity'in  directions  perpendicular  to 
the  axis.   In  consequence  of  these  un 
equal  elasticities  the  transmitted  wave 
is  divided   into  two,  which  advance 
with  unequal  velocities  ;   hence  the 
phenomena  of  double  refraction. 

The  two  parts  into  which  a  ray  is  divided  do  not  move  according: 
to  the  same  law.  Cue  follows  both  the  laws  of  n -fraction  already 
explained  ;  it  is  called  the  ordinary  ray.  The  oilier  docs  not,  as  j. 
general  thing,  follow  either  of  those  laws:  it  is  called  the  extra 
ordinary  ray.  "When  transmission  takes  place  in  the  direction  of 
the  axis,  the  two  rays  coincide  ;  when  in  a  plane  perpendicular  to 
the  axis,  the  two  ra}'s  are  most  separated. 

The  class  of  bodies  to  which  Iceland  spar  belongs  have  but  one  d'rcf- 
tion  in  which  the  refracted  rays  coincide;  these  are  called  ?mVmV^. 
There  are  bodies  that  have  two  such  directions ;  these  are  called  Unx  'nl. 


Fij*.  I79a. 


What  is  double  retraction  ?     W/iaz  it  Iceland  spar?    Wliy  does  it  separate  rays 
't  n  to  tiro  parts  ?    What,  is  th e  ordinary  ray  f    Extraordinary  ? 


KEPRACTIOX    OF    LIGHT. 

If  a  beam  of  light  be  transmitted  through  a  crystal  of  Iceland 
spar,  the  parts  into  which  it  is  divided  are  of  equal  intensity.  If  one 
of  these  parts  be  transmitted  through  a  second  crystal,  the  parts  into 
•which  it  is  divided  are  of  unequal  intensity,  and  the  degree  of 
inequality  depends  on  the  relative  positions  of  the  crystals.  Hence, 
light  that  has  been  doubly  refracted,  differs  from,  common  light; 
it  is  polarized. 

The  vibrations  that  constitute  light  are  transversal,  that  is,  they  are 
perpendicular  to  the  direction  of  propagation.  In  common  light  the 
vibrations  take  place  in  every  possible  direction  consistent  with  this 
law  ;  in  polarized  light  they  take  place  in  lines  perpendicular  to  a 
single  plane,  called  the  plane  of  polarization. 

Light  is  best  studied  by  allowing  it  to  fall  perpendicularly  on  a 
plate  of  tourmaline,  cut  parallel  to  the  axis  of  the  crystal.  Such  a 
plate  allows  no  vibrations  to  pass  except  they  be  parallel  to  the  axis. 
Hence  the  emergent  beam  is  polarized.  Let  such  a  beam  fall  per 
pendicularly  on  a  second  plate,  similar  to  the  first.  If  the  axes  of 
these  plates  are  parallel,  the  entire  beam  is  wholly  transmitted;  if 
the  axes  are  perpendicular  to  each  other,  the  beam  is  wholly  inter 
cepted  ;  if  the  axes  are  oblique  to  each  other,  the  beam  is  partially 
transmitted  and  partially  intercepted. 

If  the  rays  that  have  passed  through  a  crystal  of  Iceland  spar  be 
tested  by  a  plate  of  tourmaline,  it  is  found  that  they  are  polarized  in 
planes  which  are  perpendicular  to  each  other. 

Light  may  be  polarized  by  reflection.  We  have  seen,  when  light 
falls  on  a  surface  separating  two  media,  that  it  is  separated  into  two 
parts,  one  of  which  is  refracted  and  the  other  reflected.  When 
these' two  parts  are  perpendicular  to  each  other,  the  reflected  ray 
is  polarized  in  a  plane  normal  to  the  reflecting  surface. 

Light  may  also  be  polarized  by  refraction  by  an  ordinary  medium. 
The  plane  of  polarization  is  then  perpendicular  to  that  of  the  re 
flected  ray. 

The  mode  of  vibration  in  polarized  light  may  be  illustrated  by  a 
rope  pressing  between  two  horizontal  slats  of  a  board  fence,  the  first 
end  being  fastened  on  one  side  of  the  fence,  and  the  second  end  being 
held  by  the  hand  on  the  other  side.  If  the  hand  be  moved  rapidly 
in  any  direction,  the  wave  motion  will  be  reduced  to  a  single  plane 
after  passing  the  fence. 


What  is  polarized  light  ?    The  plane  of  polarization  ?    Action  of  plate  of  tour 
maline?    Use  as  an  analyzer  of  polarized  light  ?    Polarization  by  reflection,  f 


292 


POPULAR    PHYSICS. 


Fig.  180 


Media   with  parallel  Faces. 

292.     When   a  ray  of  light, 
Lin,  Fig.  180,  fhlls  upon  a  me 
dium   bounded  by  plane  faces, 
as  a  plate  of  glass,  for  example, 
it  is  refracted  towards  the  nor 
mal  and  passes  through  the  plate 
in   some   direction,   inn /    here 
it   is   refracted    as    much  from 
the  normal  as  it  was  towards 
it  in  the  first  instance,  and  the 
ray   emerges  in   the    direction 
no,  parallel  to  Lm.     The  two  refractions  do  not  change  the 
direction  of  the  ray,  but 
simply  shift  it  slightly  to 
one    side   or    the    other. 
Hence,  in  looking  through 
a  window,  we  do  not  see 
the  direction    of  objects 
changed  by  the  interven 
ing  glass. 

Prisms. 


A  PmsH  is  a  re 
fractive  medium  bounded 
by  plane  faces  intersect 
ing  each  other. 

Fig.  181  represents  a 
prism  mounted  for  opti 
cal  experiments.  It  con 
sists  of  a  piece  of  glass 
with  three  plane  faces, 


Fig.  181. 


(  292.)  Is  a  ray  of  light  bent  from  its  course  in  passing  through  a  medium  with 
parallel  faces?    Explain  the  phenomenon.    ( 293.)  What  is  a  Prism  ?    Explain  it. 


IIEFRACTTON     OF     LIGHT. 


293 


meeting  in  parallel  lines  called  edges.  It  is  placed  on  a  stand 
so  that  it  can  be  elevated  or  depressed,  and  it  also  is  capable 
of  being  turned  around  an  axis  parallel  to  the  edges,  by 
means  of  a  button  shown  on  the  left. 

Prisms  produce  upon  light  which  traverses  them,  two 
remarkable  effects:  1st,  a  considerable  deviation;  2d,  a 
decomposition  of  light  into  its  elements. 

These  effects  are  simultaneous,  but  we  shall  at  present 
only  consider  the  first  one,  leaving  the  second  to  be  studied 
hereafter  under  the  name  of  dispersion. 


Course   of  Luminous   Rays   in   a  Prism. 

294.     In  order  to  follow  the  course  of  a  ray  of  light  in 
passing  through  a  prism,  let  nmo,  Fig.  182,  represent  a  sec 
tion  of  a  prism  made  by  a  plane  perpendicular  to  the  edges. 
A    ray    of    light, 
La,    falling    upon     »..r 
the    face,    nm,    is  ""--^ 

refracted  towards 
the  normal,  and 
passes  through  the 
prism  in  the  direc 
tion,  ab ;  here  it 
falls  upon  the  sec 
ond  face,  mo,  and 
is  again  refracted,  Fig- 182. 

but  this  time  from 

the  normal,  and  emerging  into  the  air,  takes  the  direction, 
be.  An  eye  situated  at  c,  refers  the  object,  L,  backwards 
along  the  ray,  cb,  so  that  it  appears  to  be  situated  at  r.  The 
total  deviation  is  the  angle  between  its  original  direction, 
La,  and  its  final  direction,  cr. 

We  see  from  the  figure  that  the  ray  is  bent  from  the 


What  effect  has  a  prism  on  light? 
prism. 


(  294.)  Explain  the  course  of  a  ray  through  a 


294 


POPULAR     PHYSICS. 


edge  in  which  the  refracting  faces  meet,  that  is,  it  is  bent 
towards  the  thick  part  of  the  prism ;  this  deviation  has  the 
effect  to  make  the  object  appear  as  though  thrown  towards 
that  edge.  The  angle,  nmo,  is  called  the  refracting  angle 
of  the  prism. 


Fig.  183. 


Fig.  183  shows  the  manner  of  displacement,  caused  by 
viewing  an  object  through  a  prism.  If  the  prism  is  vertical, 
the  displacement  is  towards  the  right  or  left,  according  to 
the  position  of  the  refracting  angle. 


Lenses. 

295.     A   LENS    is    a   refracting    medium,    bounded   by 
curved  surfaces,  or  by  one  curved  and  one  plane  surface. 

Which  way  is  the  ray  bent?    Explain  Fig.  183     (  C95-)  "What  is  a  Lens? 


LENSES. 


Lenses  are  usually  made  of  glass,  and  are  bounded  by 
spherical  surfaces,  or  by  one  spherical  and  one  plane  surface. 
The  surfaces  are  made  spherical,  because  they  are  more 
easily  wrought  by  the  glass  grinder. 


Fig.  184. 


Fig.  184  represents  a  side  view,  and  Fig.  185  represents 
a  front  view  of  a  lens,  bounded  by  two  spherical  surfaces. 

Classification  of  Lenses. 

296.  Lenses  arc  divided  into  six  classes,  according  to 
the  nature  and  position  of  the  bounding  surfaces,  sections 
of  which  are  shown  in  Figs.  186  and  187. 

The  first  three,  represented  in  Fig.  186,  are  thicker  in  the 
middle  than  at  their  edges.  These  converge  or  collect  rays 
of  light,  and  are  called  convergent  lenses. 

The  last  three  are  thinner  in  the  middle  than  at  their 
edges.  These  diverge  or  scatter  rays  of  light,  and  are  called 
divergent  lenses. 

Of  what  are  lenses  made  ?  (296.)  How  many  kinds  of  lenses  aro  there?  What 
arc  convergent  lenses  ?  Divergent  lenses  ? 


296 


TOPULAK     PHYSICS. 


These  different  lenses  are  named  and  described  in  tba 
following  definitions : 

1.  The  double  convex  lens,  M,  bounded  by  two  convex 
surfaces. 


Fig.  186. 


2.  The  plano-convex  lens,  N~,  bounded  by  one  convex  and 
one  plane  surface. 

3.  The  meniscus,   0,  bounded  by  one  concave  and  one 
convex  surface,  the  concave  surface  being  the  least  curved. 

4.  The  double  concave  lens,  P,  bounded  by  two  concave 
surfaces. 

5.  The  plano-concave  lens,  Q,  bounded  by  one  concave 
and  one  plane  surface. 

6.  The  concavo-convex  lens,  7?,  bounded  by  one  concave 
and  one  convex  surface,  the  concave  surface  being  the  most 
curved. 

In  studying  the  effect  of  these  lenses,  it  will  be  sufficient 
to  consider  the  double  convex  and  the  double  concave  lenses 
as  specimens  of  the  classes  to  which  they  belong,  the  former 
representing  the  convergent,  and  the  latter  the  divergent 
classes. 


Name  and  describe  the  six  kin:ls  of  lenses  separately.    What  two  are  tak^n  a? 
specimens  1 


LENSES. 


Definitions  of  Terms. 

297.  The  centres  of  the  bounding  surfaces  of  a  lens  are 
called  Centres  of  Curvature;  thus,  in  Fig.  188,  c,  and  C, 
are  centres  of  curvature. 

In  the  double  convex  lens  the  centre  of  curvature  of  each  surface 
is  on  the  opposite  side  of  the  lens :  in  the  double  concave  lens  the 
reverse  is  the  case.  In  the  meniscus  and  the  concavo-convex  lens, 
bolh  centres  are  on  the  same  side  of  the  lens.  In  the  plano-convex 
and  the  plano-concave  lens,  the  centre  of  curvature  of  the  plane 
surface  is  at  an  infinite  distance,  and  in  a  perpendicular  to  the  plane 
surface  at  its  middle  point. 


Fig.  1S8 

The  straight  line  through  the  centres  of  curvature  is 
called  the  axis  of  the  lens;  thus,  in  Fig.  188,  XY  is  the 
axis. 

It  is  demonstrated  in  higher  optics,  that  there  is  always 
one  point  on  the  axis  of  a  lens,  such  that  the  rays  of  light 
passing  through  it,  are  not  deviated  by  the  lens.  This 
point  is  called  the  optical  centre,  and  is  of  much  use  in  the 
construction  of  images. 

In  practice  it  is  usual  to  make  the  surfaces  which  bound 
double  convex  and  double  concave  lenses,  equally  curved. 


(297)  What  are  the  Centres  of  Curvature  of  a  lens?  Where  are  they  in  tit  a 
tltn'J'l? -con-rex  lens?  Double  concave?  Meniscus?  Plano-concave  and  jjlano- 
ronTPv?  What  is  the  axis?  What  is  the  "optical  centre?  Its  use?  In  practie:-, 
iiow  are  the  curvatures  of  the  surfaces? 


2GS  POPULAR    PHYSICS. 

When  this  is  the  case,  as  we  shall  suppose  in  what  follows, 
the  optical  centre  is  on  the  axis,  and  midway  between  the 
two  surfaces  of  the  lens;  thus,  in  Fig.  188,  0  is  the  optical 
centre,  and  any  ray,  HK,  passing  through  it,  is  not  deviated 
by  the  lens. 

To  find  a  normal  at  any  point  of  the  surface  ot  a  lens,  we 
draw  a  line  from  that  point  to  the  corresponding  centre  of 
curvature  ;  thus,  m  C  and  no,  are  normals  at  the  points 
m  and  n. 

Action   of  Convex  Lenses  on  Light. 

29§.  When  a  ray  of  light  falls  upon  one  surface  of  a 
double  convex  lens,  it  is  refracted  towards  the  normal, 
passes  through  the  lens,  is  again  incident  upon  the  second 
surface,  and  is  refracted  from  the  normal.  This  action  is 
entirely  analogous  to  that  of  a  prism,  the  deviation  being 
towards  the  thicker  portion  in  both  cases.  In  fact,  if  we 
suppose  planes  to  be  drawn  tangent  to  the  surfaces  at  the 
points  of  incidence  and  emergence,  they  may  be  regarded 
us  the  faces  of  a  prism  through  which  the  ray  passes. 

Principal   Focus. 

299.  If  a  beam  ot  light,  parallel  to  the  axis,  falls  upon  a 
lens,  it  will  be  collected  by  refraction  in  a  single  point.  This 


Where  is  the  optical  centre  in  this  case?  How  do  you  find  a  normal?  (298.) 
Kvi'l.dn  the  action  of  a  convex  lens  on  light.  (299.)  What  is  the  principal 
focus  .' 


LENSES.  299 

point  is  called  the  principal  focus,  and  its  distance  from 
the  lens  is  called  the  principal  focal  distance. 

The  course  of  the  rays  is  indicated  in  Fig.  189,  in  which 
the  rays  parallel  to  CX,  are  brought  to  a  focus  at  F. 
Here,  F  is  the  principal  focus. 

tt  is  to  be  observed  that  the  rays  \vill  not  be  brought  accurately 
1o  a  focus,  except  in  the  case  in  which  the  surface  of  the  lens  is 
small,  when  compared  with  that  of  the  whole  sphere  of  which  it 
forms  part.  This  scattering  of  the  rays  from  a  focus  is  called 
spherical  aberration.  It  is  remedied  in  practice  by  covering  up  a 
part  of  the  surface  on  which  light  falls,  by  a  paper  cover  with  an 
aperture  in  its  centre. 

Had  the  rays  fallen  upon  the  other  side  of  the  lens,  they  would 
have  been  brought  to  a  focus  as  far  to  the  right  of  the  lens,  as  F  is 
to  the  left  of  it. 

Conjugate  Foci. 

3OO.  CONJUGATE  Foci  are  any  two  points  so  situated 
on  the  axis  of  a  lens,  that  a  pencil  of  light  coming  from  one 
is  brought  to  a  focus  at  the  other.  That  from  which  the 
light  actually  comes  is  called  the  radiant. 

In  Fig.  191,  a  pencil  of  rays,  coming  from  L,  is  brought 
to  a  focus  at  /,  had  the  light  come  from  /,  it  would  have 
been  brought  to  a  focus  at  L\  L  and  I  are  conjugate  foci, 
and  in  the  case  figured,  L  is  the  radiant. 

When  the  radiant  is  at  an  infinite  distance,  the  rays  are 
parallel,  and  the  corresponding  focus  is  at  F '•  this  is  the 
principal  focus.  As  we  have  already  seen,  there  are  two 
such  foci,  one  on  each  side  of  the  lens.  It  will  be  sufficient 
for  our  purpose  to  suppose  the  light  to  come  from  the  right, 
in  which  case  the  principal  focus  is  on  the  left,  at  F. 


Principal  focal  distance?  Explain  the  course  of  the  rays.  What  is  spherical 
aberration?  How  remedied?  (300.)  What  are  Conjugate  Foci?  What  is  the 
radiant?  Illustrate.  When  the  radiant  is  at  an  infinite  distance,  where  is  the  con 
jugate  focus? 


300  POPULAR     PHYSICS. 

When  the  radiant  is  anywhere  on  the  axis  at  a  greater 
distance  than  the  principal  focal  distance,  the  corresponding 
focus  will  also  be  at  a  greater  distance  from  the  lens  than 
the  principal  local  distance,  as  shown  in  Fig.  190. 

Fi?.  190. 


Fig.  191. 

If  the  radiant  approach  the  lens,  the  corresponding  focus 
will  recede  from  it,  as  is  shown  in  Fig.  191. 

If  the  radiant  is  at  the  principal  focal  distance,  the  re 
fracted  rays  will  be  parallel,  that  is,  the  corresponding  focus 
will  be  at  an  infinite  distance,  as  is  shown  in  the  upper  dia 
gram,  (Fig.  192). 

If  the  radiant  is  still  nearer  the  lens,  the  rays  will  diverge 
after  deviation,  and  will  only  meet  the  axis  on  being  pro 
duced  backwards,  in  which  case  the  focus  is  virtual,  as  is 
shown  in  the  lower  diagram,  (Fig.  193).  In  this  diagram 
J*  is  the  radiant,  and  I  the  virtual  focus. 

Thus  far  we  have  supposed  the  radiant  to  be  situated  on 
the  principal  axis;  if  it  is  on  any  line  through  the  optical 
centre  not  much  inclined  to  the  axis,  the  corresponding 

When  the  radiant  is  at  a  distance  greater  than  the  principal  focal  distance,  where 
is  the  conjugate  focus?  When  the  radiant  approaches  the  lens?  When  at  the  prin 
cipal  focus?  If  still  nearer  (ho  lens?  •Suppose  Ui2  radian'  not  on  the  axis  ? 


LEASES.  301 

focus  will  be  on  that  line,  and  the  laws  which  regulate  the 
positions  of  conjugate  foci,  already  considered,  will  be  ap 
plicable. 

These  principles  are  of  use  in  the  discussion  of  images 
formed  by  lenses. 

Fig   192. 


Fig.  193. 
Formation  of  Images  by  Convex  Lenses. 

3O1.  If  an  object  be  placed  in  front  of  a  lens,  each  point 
cf  it  may  be  regarded  as  a  radiant  sending  out  a  pencil  of 
rays.  Each  pencil  is  brought  to  a  locus  somewhere  behind 
the  lens.  The  assemblage  of  these  foci  makes  np  a  picture 
of  the  object.,  which  is  called  its  image.  When  the  object  is 
at  a  greater  distance  from  the  lens  than  the  principal  focal 
distance,  the  image  will  be  real  and  inverted.  The  course 
of  the  rays  is  shown  in  Fig.  194.  The  image  is  real,  as  may 
be  shown  by  throwing  it  upon  a  screen ;  so  long  as  the 
image  is  real,  it  is  inverted,  as  may  be  seen  by  allowing  it 
to  fall  upon  a  screen,  or  it  may  otherwise  be  shown  from 
the  fact  that  the  axis  of  each  pencil  passes  through  the  op 
tical  centre  ;  hence  the  image  of  each  point  is  on  the  opposite 
side  of  the  axis  from  the  point. 


(301.)  Explain  the  formation  of  an  imago  by  a  Ion?. 


302  POPULAR     P1IYCICS. 

With  respect  to  the  size  of  the  image  in  this  case,  it  may 
be  either  greater  or  smaller  than  the  object.  When  the 
object  is  farther  from  the  lens  than  twice  the  principal  focal 
distance,  the  image  is  smaller  than  the  object ;  when  the 
object  is  at  twice  the  focal  distance,  the  image  is  of  the 


fig.  194. 


same  size  as  the  object ;  when  the  distance  is  less  than  twice 
the  principal  focal  distance,  and  greater  than  the  principal 
focal  distance,  the  image  is  greater  than  the  object. 

These  principles  may  be  shown  experimentally  as  follows: 

Let  a  convex  lens  be  placed  in  a  dark  room,  and  suppose  its  prin 
cipal  focal  distance  to  have  been  determined  by  means  of  a  beam  of 
solar  rays.  Let  a  candle  be  placed  in  front  of  the  lens,  and  a  screen 
behind  it  to  receive  its  image,  as  shown  in  Fig.  195. 

When  the  distance  of  the  candle  from  the  lens  is  more  than  twice 
the  principal  focal  distance,  its  image  will  be  less  than  the  object: 
and  the  more  remote  the  candle,  the  less  will  be  its  image. 

If  the  candle  be  moved  towards  the  lens,  its  image  will  grow  larger. 
nnt.il,  at  twice  the  principal  focal  distance,  the  size  of  the  image  and 
object  will  be  equal. 

If  the  candle  be  moved  stilt  nearer,  the  size  of  the  image  will  be 


How  does  the  size  of  the  image  compare  with  that  of  the  object  in  different  cases? 
Explain  in  detail  the  method  of  illustrating  the  foregoing  principles  by  experi 
ment. 


LEASES. 


303 


increased,  that  is,  it  will  become  greater  than  the  object,  as  is  shown 
in  Fig.  196. 

If  the  distance  of  the  object  does  not  become  smaller  than  the 
principal  focal  distance,  the  image  will  be  inverted,  as  is  shown  in 
Figs.  195  and  196. 


If  the  object  approach  still  nearer  the  lens,  that  is.  if  its  distance 
becomes  less  than  the  principal  focal  distance,  the  image  will  in 
crease,  it  will  become  erect,  and  furthermore  it  will  be  virtual.  The 
course  of  the  rays  in  this  case  is  shown  in  Fig.  197.  Here,  ab  is  the 
object,  and  AB  is  its  image,  which  can  only  be  seen  by  looking 
through  the  lens. 

In  this  case  the  lens  becomes  what  is  called  a  single  microscope. 

When  the  object  is  at  the  principal  focal  distance  from  the  lens, 
the  image  is  infinite  ;  that  is.  it  disappears. 

What  is  a  single  microscope  ?    Illustrate. 


304 


POPULAR     PHYSICS. 


The  phenomena  just  described  may  be  observed  by  looking  through 
a  convex  lens  at  the  letters  on  a  printed  page.  When  the  letters  are 
at  a  short  distance  from  the  lens,  they  are  magnified  and  erect ; 


Fig. 


LENSES.  3t)5 

on  removing  the  lens,  they  disappear  at  the  principal  focal  distance, 
and  finally  reappear  inverted  and  diminished  in  size. 

Formation   of  Images  by  Concave   Lenses. 

3©2.  Concave  lenses  being  thinner  in  the  middle  than 
:it  the  edges,  have  the  effect  to  diverge  parallel  rays.  If  the 
rays  are  already  divergent,  these  lenses  make  them  still 
more  so. 

This  is  shown  in  Fig.  198,  in  which  a  pencil  of  rays,  coming  from 
the  radiant,  L,  are  made  to  diverge,  as  though  they  proceeded  from 
u  point,  /,  nearer  the  lens.  This  point,  /,  is  the  virtual  focus,  cor 
responding  to  the  radiant,  L.  To  an  eye  situated  on  the  left  of  the 
lens,  the  light,  L,  appears  to  be  situated  at  /. 


Fig.  198. 

From  what  has  been  said,  it  is  plain  that  the  images 
formed  by  concave  lenses  are  virtual.  They  are  also  erect, 
as  in  Fig.  198. 

The  course  of  the  rays,  in  forming  an  image  in  the  case 
of  a  concave  lens,  is  shown  in  Fig.  199.  In  that  figure,  AB 
represents  the  object.  A  pencil  of  rays,  coming  from  A,  is 
deviated  so  as  to  appear  to  come  from  a,  situated  on  a  line 
drawn  from  A  to  the  optical  centre  of  the  lens  O.  A  pencil, 
coming  from  J5,  is  deviated  so  as  to  appear  to  come  from  £, 


(302)  What  is  the  effect  of  a  divergent  lens  upon  Iteht?  Explain  Firj.  198. 
What  kind  of  images  are  formed  by  concave  lenses?  Explain  the  course  of  the  rays 
in  a  concave  lens. 


POPULAR     PHYSICS. 

on  the  lino  Bo.     Hence,  rib  is  the  iivmere  of  the  object  AB, 


and  is,  as  we  see,  smaller  than  the  object,  being  nearer  the 
optical  centre,  and  furthermore  it  is  erect. 

Burning-glasses. 

3O3.  Rays  of  heat  arc  subject  to  the  same  laws  of 
reflection  and  refraction  as  rays  of  light.  When  a  beam  of 
solar  light  falls  upon  a  convex  lens,  there  is  not  only  a  con 
centration  of  light  at  the  focus,  but  of  heat  also. 

The  heat  concentrated  is  so  great  as  to  inflame  combustible  bodies, 
such  as  paper,  cloth,  wood,  and  the  like.  In  the  case  of  large  lenses, 
the  heat  becomes  sufficiently  powerful  to  fuse  metals.  This  property 
of  lenses  has  been  used  to  procure  fire  ;  the  lens  in  this  case  is  called 
a  burning-glass.  Lenses  carelessly  exposed  may  sometimes  cause 
dangerous  results,  by  setting  fire  to  inflammable  materials.  This 
effect  may  result  from  spherical  vessels  of  glass  filled  with  water, 
which  possess  all  the  properties  of  lenses. 

Do  roncavo  lenses  magnify  or  diminish  objects?    (303.)  How  are  rays  of  livut 
offjc'etl  by  lenses?     Whnt  is  n  burning-rfl-ifs*?    Evpl-iin  itt  action. 


LENSES.  307 

A  curious  application  of  this  principle  is  shown  in  Fig.  200.  A 
lens  is  arranged  with  its  axis  in  the  meridian,  so  that  its  principal 
focus  shall  fall  upon  the  vent  of  a  small  cannon  When  the  sun 


Fig.  200. 

crosses  the  meridian,  Ihe  rays  arc  concentrated  upon  the  vent,  and 
if  the  gun  has  been  loaded  and  primed  beforehand,  it  will  be  dis 
charged  at  midday. 

Light-houses. 

3O4.  LIGHT-HOUSES  are  towers,  erected  along  the  coast, 
upon  the  tops  of  which  are  lanterns.  These  lanterns  are 
lighted  at  night  as  guides  to  mariners. 

One  of  the  most  famous  light-houses  of  antiquity  was  that  on  tie 
little  Island  of  Pharos,  near  Alexandria,  in  Egypt.  From  the  loca 
tion  of  this  light-house  the  French  derive  the  name  pharo.  which 
they  apply  1o  all  light-houses.  In  former  times  light-houses  were 
illuminated  by  fires  built  with  wood,  coal,  or  some  bituminous  sub 
stances. 


Explain  Fig  200.    (304.)  What  is  a  light-house?     Give  an  account  of  the  an 
cient  Hght-7iO"ftcs. 


308 


POPULAR    PHYSICS. 


These  methods  of  illumination  were  afterwards  replaced  by  oil 
lamps  placed  in  the  foci  of  concave  reflectors,  which  served  to  con 
centrate  the  rays,  and  thus  to  heighten  their  illuminating  effect. 
But  the  reflectors,  being  made  of  metal,  were  soon  tarnished,  and 
the  light  afforded  became  feeble. 

In  1822,  FRESNEL,  already  distinguished  by  his  discoveries 
in  optics,  and  by  his  researches  on  the  wave  theory  of  light, 
invented  a  new  system  of  illumination,  which  is  now  being 
aoopted  in  all  civilized  countries. 


Fig.  201. 

Abandoning  the  reflectors,  which  became  tarnished  by  the  influence 
of  sea  fogs,  he  substituted  for  them  plano-convex  lenses,  in  the 
principal  foci  of  which  he  placed  powerful  lamps  with  four  con 
centric  wicks,  each  of  which,  for  the  quantity  of  oil  consumed,  and 
the  amount  of  light  given  out,  was  found  to  be  equivalent  to  seven 
teen  carcel-lamps.  The  difficulty  of  constructing  large  plano-convex 
lenses,  together  with  their  great  absorption  of  light,  led  finally  to  the 
adoption  of  a  particular  system  or  lenses,  known  as  echelon  lenses. 

These  lenses  will  be  understood  by  examining  Figs.  201  and  202  : 


Explain  the  principle  of  reflectors.    What  modification  did  FRESNEL  introduce  ? 
Explain  the  echelon  lens. 


LENSES.  309 

Fig.  201  shows  a  front  view,  and  Fig.  202  a  section  or  profile  of  an 
echelon  lens. 

A  lens  of  this  kind  consists  of  a  plano-convex  lens,  A.  about  a  foot 
in  diameter,  around  which  are  disposed  several  annular  lenses,  which 
are  also  plano-convex,  and  whose  curvature  is  so  calculated  that 
each  one  shall  have  the  same  principal  focus  as  the  central  lens,  A. 

A  lamp.  L.  being  placed  at  the  principal  focus  of  this  refracting 
system,  as  shown  in  Fig  202.  the  light  emanating  from  it  is  refracted 
into  an  immense  beam,  RC,  of  parallel  rays. 


Fig.  203. 


Explain  the  reflectors  used  l>y  FRESNEL. 


310  POPULAIJ   niYcirs. 

Besides  this  refracting  system,  several  ranges  of  reflectors,  mn.  are 
so  disposed  as  to  reflect  such  light  as  would  otherwise  be  lost,  to 
increase  the  beam  of  light  formed  by  refraction. 

By  this  double  combination,  an  immense  beam  of  light  is  afforded, 
which  renders  the  light  visible  for  fifteen  or  twenty  leagues  :  but  this 
l.earn  is  only  visible  in  a  single  direction.  To  remedy  this  defect, 
FRESNEL  united  eight  systems  similar  to  that  just  described,  which 
combination  presents  the  appearance  of  a  pyramid  of  glass,  nine  or 
ten  feet  in  height. 

Fig.  203  represents  a  section  of  the  lantern  of  a  light-house  of  the 
first  order,  which  was  ;  dually  constructed  by  M.  SAUTTER,  and 
exhibited  at  the  great  "  Universal  Exposition"  of  France,  in  1855. 

In  order  to  illuminate  all  points  of  the  horizon,  the  system  is  made 
to  revolve  on  a  vertical  axis  by  clock-work.  The  clock-work  is 
shown  at  M  in  the  figure,  and  the  weight  at  P.  To  prevent  friction 
the  system  turns  upon  six  wheels,  or  rollers,  shown  in  the  figure  to 
the  left  of  M. 

In  consequence  of  this  rotation  an  observer  at  any  point  will  see 
eight  flashes  of  light  during  one  revolution,  which  arc  followed  by 
as  many  intervals  of  darkness,  called  eclipses.  By  suitably  regulating 
the  number  of  revolutions  in  any  given  time,  different  light-houses 
may  be  distinguished  from  each  other. 


IV. — DECOMPOSITION    OF     LIGHT. — COLORS    OP    BODIES. 

Solar  Spectrum. 

305.  If  a  beam  of  sunlight  pass  through  a  prism,  it  is 
bent  from  its  course  and  at  the  same  time  is  spread  out 
into  a  brilliantly  colored  band,  called  the  solar  spectrum. 
The  spreading  of  the  rays  is  called  dispersion  /  it  is  caused 
by  unequal  refrangibility  of  the  different  colored  rays. 
The  angular  dispersion  of  rays  is  different  for  different 
media. 

How  far  is  a  FRESNEL  light  visible?  How  are  all  points  of  the  horizon  illumi 
nated?  Explain  flashes  and  eclipses.  (305.)  What  is  the  solar  spectrum?  What 
is  dispersion. 


DECOMPOSITION    OF    LIGHT   AND    COLOK. 


311 


The  method  of  forming  a  spectrum  is  shown  in  the  figure.  The 
beam  of  light  that  enters  a  hole  in  the  shutter  falls  on  a  prism  whose 
refracting  edge  is  turned  downward ;  the  whole  beam  is  bent  up 
ward,  and  at  the  same  time  its  elements  are  dispersed  so  as  to  form 
the  elongated  spectrum  seen  on  the  screen. 


Fig.  21)5. 

This  spectrum  consists  of  almost  an  infinite  number  of  rays  of 
different  tints,  but  it  is  customary  to  consider  only  seven  principal 
colors.  These,  in  the  order  of  refrangibility,  are  as  follows  :  1°,  rc3, 
at  r;  2°,  orange,  at  o;  3J,  yellow,  at  y  ;  4°, green,  at  rj ;  5°,  blue,  at  b; 
G°,  indigo,  at  i;  and  7°,  violet,  at  v. 

Besides  the  colored  portions  of  the  solar  spectrum,  there  is  an  in 
visible  portion  below  the  red,  where  the  heat  is  greater  than  any 
where  else,  and  another  portion  above  the  violet,  where  the  chemical 
effect  is  greater  than  anywhere  else.  The  corresponding  rays  are 
called  heat  rays  and  actinic  or  chemical  rays. 


Name  the  colors  in  order  of  refrangibility .    What  are  heat  rays  ?     Chemical  or 
actinic  rays  ? 


oLX  POPULAR    PHYSICS. 

If  a  colored  ray  of  the  spectrum  pass  through  a  hole  in  a  screen, 
and  then  fall  on  a  second  prism,  it  is  deviated  as  before,  but  there  is 
no  further  change  of  color ;  hence,  the  colors  of  the  spectrum  art- 
said  to  be  simple. 

The  wave  lengths  corresponding  to  different  colored  rays  have 
been  measured,  and  it  is  found  that  for  red  rays  it  is  about  the  forty 
thousandth  part  of  an  'inch,  and  for  violet  rays  it  is  no  more  than 
the  sixty  thousandth  part  of  an  inch.  For  the  invisible  rays  of  heat 
the  wave  length  is  greater  than  it  is  for  red  light ;  for  the  invisible 
actinic  rays  the  wave  length  is  less  than  it  is  for  violet  light.  The 
phenomena  of  dispersion  indicate  that  shorter  waves  are  more 
retarded  than  longer  ones  in  passing  through  a  medium  ;  hence,  the 
rays  near  the  heat  end  of  the  spectrum  are  least  refracted,  and  those 
near  the  actinic  end  are  most  refracted.  » 

Color  in  light  corresponds  to  pitch  in  sound.  The  colors  near  the 
heat  end  of  the  spectrum  correspond  to  the  graver  sounds,  and  those 
near  the  actinic  end  to  the  more  acute  sounds.  The  range  of  visible 
colors  is  greater  than  that  of  audible  sounds.  The  range  of  visible 
colors  is  scarcely  one  octave,  that  of  audible  sounds  is  more  than  ten 
octaves. 


Fraunhofer's  Lines,— The  Spectroscope. 

30<3.  The  solar  spectrum  is  not  continuous;  rays  cor 
responding  to  certain  degrees  of  refrangibility  are  wanting ; 
hence,  it  is  crossed  at  intervals  by  dark  lines.  These  are 
seen  to  best  advantage  in  a  spectrum  formed  by  passing  a 
beam  of  sunlight  through  a  narrow  slit,  and  then  decom 
posing  it  by  a  prism  whose  edges  are  parallel  to  the  slit. 
The  prism  should  be  of  flint  glass  and  free  from  flaws. 

The  dark  lines  of  the  solar  spectrum  were  noticed  by 
WOLLASTOX  as  early  as  1802,  but  they  were  first  studied 
and  mapped  by  Fraunhofer  in  1814;  from  that  fact  they 
have  been  called  FrannJiofe^s  lines. 

Fraunhofer's  chart  contains  between  five  and  six  hundred  lines 
irregularly  distributed.  In  it  the  most  prominent  lines  are  designated 

What  is  the  wave  length  of  red  f  Of  violet  ?  What  relation  is  there  between  color 
and  pitch  ?  (306.)  What  arc  Fraunhofer's  lines  ? 


DECOMPOSITION    OF    LIGHT    AND    COLOR.  OiO 

by  letters,  and  these  serve  as  points  of  comparison  to  which  others  may 
be  referred.  The  line  marked  A  is  at  the  beginning,  and  B  is  near 
the  middle  of  the  red  space  ;  C  is  a  well-marked  line  near  the  bound 
ary  of  the  red  and  orange;  D  consists  of  two  strong  lines  close 
together  in  the  orange  ;  E  consists  of  broad  lines  in  the  green,  the 
middle  one  being  the  strongest ;  F,  6r,  and  H,  are  well-marked  lines, 
F  being  in  the  blue,  G  in  the  indigo,  and  H'm  the  violet.  Between 
A  and  B  is  a  band  of  lines  named  a,  and  between  E  and  F  are  three 
strong  lines  called  6,  the  two  farthest  from  E  being  close  together. 

Fraimhofer  counted  nine  lines  between  B  and  C ;  thirty  between 
C  and  D  ;  eighty -four  between  D  and  E ;  seventy-Jive  between  E  and 
F;  one  hundred  and  eighty-Jive  between  Fand  G  ;  and  one  hundred 
and  ninety  between  G  and  H.  Recent  observations  have  increased 
the  number  of  dark  lines  till  they  are  now  counted  by  thousands. 

Fraunhofer  found  the  spectra  of  the  fixed  stars  to  be  crossed  by 
dark  lines,  but  the  lines  are  differently  arranged  in  the  different 
stars,  and  in  none  are  they  arranged  as  in  the  solar  spectrum. 
Recently  the  range  of  observation  has  been  vastly  increased,  and  on 
the  results  of  these  examinations  a  new  branch  of  science  has  been 
founded,  called  spectrum  analysis. 

The  instrument  used  for  forming  and  examining  the 
spectra  of  bodies  is  called  a  spectroscope.  It  usually  con 
sists  of  three  parts:  a  collimator,  a  train  of  prisms,  and  a 
telescope. 

The  collimator  is  used  to  form  a  thin  beam  of  parallel  rays,  and 
consists  of  a  narrow  slit  and  a  double  convex  lens ;  the  slit  is  formed 
by  two  jaws  of  metal  that  can  be  moved  to  and  from  each  other,  so 
as  to  give  as  narrow  an  opening  as  may  be  desired ;  the  lens  is 
behind  the  slit,  and  at  a  distance  from  it  equal  to  its  principal  focal 
distance ;  hence,  it  renders  the  rays  that  pass  through  it  parallel  to 
each  other.  The  train  of  prisms  serves  to  disperse  the  light ;  it  con 
sists  of  any  number  of  prisms  having  their  edges  parallel  to  the  slit, 
and  so  placed  that  the  light  shall  pass  through  fiem  all  in  succes 
sion.  The  telescope  is  used  to  view  the  spectrum  formed,  and  is 
usually  provided  writh  a  micrometer  for  measuring  the  distances 
between  the  lines  of  the  spectrum  ;  it  admits  of  a  certain  amount  of 
angular  motion,  so  that  it  can  be  made  to  embrace  in  succession 
every  part  of  the  spectrum. 

Describe  Ihe  positions  of  the  principal  lines  of  the  solar  spectrum.  What  is  a 
spectroscope  ?  Of  how  many  parts  is  it  composed  ?  Describe  the  collimator. 
The  train  of  prisms.  The  telescope. 

14 


314  POPLLAI:  PHYSICS. 


Spectrum  Analysis. — Explanation  of  Fraunhofer's  Lines. 

307.  Metals  and  their  compounds  impart  characteristic 
colors  to  flames ;  thus,  sodium  and  its  compounds  impart 
a  yellow  color  to  a  Bunsen  burner;  the  compounds  of 
copper  render  it  green,  the  compounds  of  zinc  make  it 
purple,  and  the  compounds  of  strontian  give  it  a  red 
color.  These  colors  are  due  to  the  vapors  of  the  corre 
sponding  substances,  and  are  peculiar  to  those  vapors.  If 
these  or  any  other  incandescent  vapors  be  examined  with 
the  spectroscope,  their  spectra  are  found  to  consist  of 
bright  bands,  each  corresponding  to  a  definite  degree  of 
refr&ngibility.  The  number,  color,  and  position  of  the 
bands  in  every  case  are  perfectly  characteristic,  and  always 
serve  to  identify  the  body  producing  the  spectrum.  This 
mode  of  determining  the  presence  of  bodies  is  called 
spectrum  analysis. 

If  two  or  more  metals  be  vaporized  in  the  flame  at  the  same  time, 
the  bands  peculiar  to  each  are  formed  as  though  the  others  did  not 
exist.  If  a  mineral  substance  containing  many  different  metals  be 
volatilized  the  spectrum  will  show  the  bands  characteristic  of  each. 
Bunsen  and  Kirchoff  discovered  the  new  metals  Rubidium  and  Cae 
sium,  by  means  of  bands  shown  by  the  spectroscope,  which  differed 
from  those  of  all  the  metals  previously  known,  and  in  like  manner 
Mr.  Crookes  discovered  the  new  metal  Thallium. 

The  method  of  spectrum  analysis  is  exceedingly  delicate ;  the  pres 
ence  of  the  minutest  portion  of  any  substance  in  the  form  of  incan 
descent  vapor  is  instantly  made  manifest  by  its  characteristic  lines  in 
the  spectrum. 

It  has  been  shown  that  an  incandescent  solid  or  liquid 
gives  a  continuous  spectrum.  If  light  from  such  a  source 
be  transmitted  through  the  vapors  of  any  substances,  and 
then  examined  with  the  spectroscope,  the  resulting  spec- 


(307.)  What  is  spectrum  analysis?  Character  of  the  spectrum  of  an  incan- 
descen,t  vapor.  Explain  the  action  of  sodium,  copper,  zinc,  and  etrontiau  011  the 
flame  of  a  Bunsen  burner. 


DECOMPOSITION    OF    LIGHT    AND    COLOE,  315 

trum  will  be  crossed  by  dark  lines  having  the  same  position 
as  the  bright  lines  belonging  to  the  spectra  of  the  vapors. 
Hence,  it  appears  that  every  body  in  a  state  of  vapor  is 
opaque  to  the  class  of  rays  that  it  emits  when  rendered 
incandescent. 

The  principle  just  elucidated  has  been  applied  to  explain  the  dark 
'  lines  of  the  solar  spectrum.  It  is  supposed  that  the  body  of  the  sun 
is  an  incandescent  solid,  or  perhaps  a  glowing  liquid,  and  conse 
quently  that  it  emits  \vhite  light.  It  is  further  supposed  that  the 
body  of  the  sun  is  surrounded  by  a  layer  of  gaseous  matter  contain 
ing  vapors  of  various  substances,  including  many  of  the  known 
metals.  This  envelope,  called  the  photosphere,  being  at  a  lower 
temperature  than  the  nucleus,  is  in  a  condition  to  absorb  the  very 
rays  that  it  would  itself  emit,  if  it  were  incandescent.  The  absorbed 
or  missing  rays  form  the  dark  lines  of  the  spectrum.  Were  the  cen 
tral  nucleus  abolished,  the  solar  spectrum  would  be  transformed  into 
a  system  of  brilliant  bands.  These  would  correspond  to  the  bands 
of  a  spectrum  given  by  a  flame  charged  by  metallic  vapors.  They 
would  constitute  the  spectrum  of  the  solar  photosphere. 

The  following  metals  have  been  shown  to  exist  in  the  photosphere 
of  the  sun  :  viz.,  sodium,  calcium,  barium,,  magnesium,  iron,  chromium, 
nickel,  copper,  zinc,  strontium,  cadium,  cobalt,  hydrogen,  manganese, 
aluminum,  and  titanium — sixteen  in  all. 

The  spectra  of  the  fixed  stars  indicale  that  those  bodies  are  simi 
lar  in  constitution  to  our  sun,  but  the  number  and  position  of  the 
dark  lines  show  that  their  photospheres  do  not  contain  the  same 
elements  that  are  found  in  our  own  luminary. 

It  has  long  been  known  that  the  sun  is  surrounded,  during  the 
time  of  a  total  eclipse  by  a  great  number  of  irregular  rose-colored  pro 
tuberances.  These  have  been  shown  by  spectrum  analysis  to  con 
sist,  for  the  most  part,  of  incandescent  hydrogen  ;  with  it  are  mixed 
vapors  of  sodium  and  magnesium.  The  protuberances  form  part 
of  an  irregular  envelope  surrounding  the  entire  body  of  the  sun,  and 
lying  outside  of  its  photosphere.  This  layer  c.onstitutcs  what  has 
been  named  the  chromosphere,  and  within  a  few  years  a  method  has 


What  is  the  character  of  the  spectrum  of  an  incandescent  solid  ?  Action  on 
.light  transmitted  through  a  vapor.  Explanation  of  Fraunhofer"1  s  lines.  What  is 
the  constitution  of  the  sun  f  What  metals  have  been  detected  in  tlie  sun?  What  is 
the  constitution  r>f  the  fixed  tiars?  Describe  the  chromosphere. 


310 


POPULAR    PHYSICS. 


been  discovered  for  observing  its  spectrum  without  the  necessity  of 
waiting  for  a  total  eclipse. 

Heat  Rays  and  Actinic  Rays. 

307a.  The  seven  rays  enumerated  differ  in  illuminating 
power,  the  middle  rays  being  those  which  possess  the- 
greatest  illuminating  power.  That  .is,  the  most  powerfully 
illuminating  rays  lie  midway  between  the  heat  rays  and  the 
actinic  rays. 

If  a  thermometer  be  held  for  a  time  in  the  diiferent  rays, 
beginning  at  the  violet,  it  will  show  an  increase  of  heat  till 
it  comes  outside  of  the  red  rays,  where  it  is  greatest. 

The  actinic  rays  are  those  that  produce  chemical  changes. 
If  a  strip  of  paper,  prepared  with  nitrate  of  silver,  be  placed 
in  the  spectrum,  it  will  be  least  changed  in  the  red,  and  in 
passing  towards  the  violet  end,  this  change  will  increase 
till  it  becomes  the  greatest  beyond  the  violet. 

Recomposition  of  Light. 

308.  The  colors  of  the  spectrum  may  be  reunited  so  as 
to  produce  white  light. 

1.  If  it  be  acted  on  by  a  second  prism  exactly  like  the 
first,  with  its  refracting  edge  turned  in  the  opposite  direc 
tion,  it  will  be  recomposed  and  will  emerge  as  white  light. 

This  amounts  to  nothing  more  than  passing  light  through  a  medi 
um  bounded  by  parallel  plane  faces. 

2.  If  it  be  received  on  a  double  convex  lens,  as  shown  in 
Figure  206,  it  will  be  recomposed  and  an  image  will  be 
formed  free  from  color. 

The  manner  of  performing  this  experiment  is  shown  in  Fig.  206. 

(3O7a.)  Which  are  the  most  illuminating  rays  ?  How  is  their  heating  power? 
What  are  actinic  rays  ?  Which  produce  the  greatest  chemical  effect  ?  How 
shown?  (308.)  May  the  rays  of  the  spectrum  be  reunited?  First  method. 
Second  method. 


DECOMPOSITION    OF    LIGHT    AND    COLOR. 


317 


Fig.  206 

3.  If  the  decomposed  light  be  received  upon  a  concave 
mirror,  it  will  in  like  manner  be  recomposed  and  a  colorless 
image  produced. 

4.  If  a  circular  disk  of  card-board  be  painted  as  shown  in 
Fig.  207,  in  sectors,  the  colors  being  distributed  according 
to  intensity  and  tint,  as  in  the  spectrum,  it  will  be  found  on 
rotating  the  disk  rapidly  by  a  piece  of  mechanism  shown  in 
Fig.  208,  that  the  separate  colors  blend  into  a  single  one, 
which  is  a  grayish  white. 

The  color  from  any  sector  produces  upon   the   eye  an  impression 


Third  method  ?    Fourth  method  : 


318 


POPULAR     PHYSICS. 


Fig.  20T. 


Fig.  208. 


that  lasts  for  an  appreciable  length  of  time.  In  the  experiment,  the 
rotation  is  so  rapid  that  the  impressions  from  all  of  the  colors  coexist 
at  the  same  instant,  and  the  effect  is  the  same  as  though  the  colors 
were  mixed. 

That  the  impression  produced  by  light  lasts  for  an  appreciable 
length  of  time,  may  be  shown  by  whirling  a  lighted  stick  round  in  a 
circle;  it  will  present  the  appearance  of  a  continuous  circle  of  fire. 

Color  of  Opaque  Bodies. 

3O9.  The  color  of  a  body  may  be  temporary  or  perma 
nent.  Temporary  colors  arise  from  some  modification  of 
light,  of  a  transient  character. 

JTow  does  it  appear  that  the  impression  of  color  lasts  for  a  short  time  ?    H<no 
may  it  l>e  shmcn  ?    (  309.)  From  what  does  the  color  of  a  body  arise? 


DECOMPOSITION     OF    LIGHT    AND    COLOR.  319 

Thus,  by  refraction,  certain  drops  of  water  in  the  air  are  colored, 
producing  the  rainbow  ;  the  color  of  these  drops  is  due  to  their 
posifion  with  respect  to  the  eye  and  the  sun.  The  colors  of  soap- 
bubbles  are  dependent  upon  interference,  a  principle  not  yet  ex 
plained,  and  are  transitory. 

The  colors  of  finely-grooved  surfaces  are  due  to  interference.  These 
colors  are  independent  of  the  physical  constitution  of  the  body,  and 
depend  solely  upon  the  fineness  and  shape  of  the  grooves. 

The  play  of  colors  upon  mother-of-pearl  is  due  to  fine  grooves  i>r 
striae,  as  may  be  shown  by  taking  an  impression  of  a  piece  of  it  in 
white  wax ;  the  colors  of  the  wax,  thus  prepared,  are  entirely 
analogous  with  those  of  the  mother-of-pearl,  from  which  the  im 
pression  was  taken. 

With  respect  to  the  permanent  colors  of  bodies,  various 
opinions  have  been  held.  NEWTON  conceived  that  bodies 
had  the  power  of  absorbing  some  of  the  rays  of  the  spectrum, 
and  reflecting  the  remainder.  According  to  this  theory,  the 
color  of  a  body  would  be  that  arising  from  a  mixture  of  the 
reflected  rays.  Thus,  vermilion  was  supposed  to  have  the 
power  of  reflecting  the  red  rays  only,  whilst  all  of  the 
others  were  absorbed.  All  bodies  placed  in  a  red  light 
appear  red,  in  a  blue  light,  blue,  and  so  on  for  other  colors. 

ARAGO  was  of  the  opinion  that  the  colors  of  bodies  arose 
from  light  admitted  into  the  body  and  then  emitted  again, 
undergoing  certain  modifications,  ^olor  would,  according 
to  this  theory,  depend  upon  the  molecular  condition  of  the 
body.  According  to  this  view,  color  is  a  modification  of 
light,  entirely  analogous  to  that  modification  of  sound  which 
we  call  the  tone. 

ARAGO'S  theory  was  based  upon  a  difference  of  property  between 
reflected  and  refracted  light.  On  examining  the  colors  of  opaque 
bodies,  he  found  that  the  light  agreed  with  that  which  had  been 
refracted,  rather  than  with  that  which  had  been  reflected. 


Explain  temporary  color*  in  case  of  rain-drops.  Of  grooved  surfaces.  Of 
mother-of-pearl.  What  is  NEWTON'S  theory  of  colors  of  bodies  ?  What  is  ABAGO'S 
theory  ?  On  what  was  ABAGO'S  theory  based  '? 


320  POPULAR     PHYSICS. 

Colors  of  Transparent  Bodies. 

310.  All  transparent  bodies  absorb  more  or  less  of  the 
light   which    enters    them,   and   if  sufficiently   thick,  must 
appear  colored.     Their  color  is  due  to  that  part  of  the  light 
which  is  transmitted. 

Jf.  fur  example,  all  of  the  solar  rays  except  the  red  ones  are  ab 
sorbed  b/  tt  medium,  it  will  appear  red  by  transmitted  light.  Water 
when  seen  m  masses  by  transmitted  light,  appears  of  a  greenish  hue. 
Air  appears  blue;  hence  the  color  of  the  sky.  As  we  ascend,  the 
mass  above  us  becomes  smaller  and  loses  its  blue  tint.  It  is  proba 
ble  that  the  bluish  tint  of  the  heavens  is  also  in  a  measure  due  to 
reflection  from  ihe  aerial  molecules.  At  sunrise  and  sunset,  the 
rays  of  the  sun  have  to  traverse  a  great  body  of  the  atmosphere, 
which  absorbs  must  of  the  rays  except  the  red  ones.  Hence  it  is, 
that  the  sun  appears  red  at  sunrise  and  sunset. 

Complementary  Colors. 

311.  NEWYON  calls  two  colors  complementary,  when  by 
their  mixture  t  hey  produce  white. 

If  all  the  rays  of  the  spectrum  except  the  red  ones  be 
recomposed  by  a  convex  lens,  a  bluish-green  color  will 
result ;  hence,  red  and  green  are  complementary.  In  like 
manner,  it  may  be  shown  that  blue  and  orange  are  com 
plementary,  as  are  also  violet  and  yellow. 

Accidental  Images.  —  Accidental  Fringes. 

312.  A  curious  effect  of  color  upon  the  eye  is  manifest  in  the 
production  of  what  are  called  accidental  images. 

If  a  wafer  upon  a  black  ground  be  viewed  intently  for  some  time, 
until  the  nerve  of  the  eye  becomes  fatigued,  and  the  eye  be  then 
directed  to  a  sheet  of  white  paper,  an  image  of  Ihe  wafer  will  be 
seen  upon  the  paper,  whose  color  is  complementary  to  that  of  the 


(  31O.)  To  what  is  the  color  of  transparent  holies  due?  Illustrate  '?'  examples. 
(311  )  What  are  complementary  co'.^'-s?  What  is  t1^  complement  of  red  ?  Of 
green  ?  Of  blue  ?  Of  orange  ?  (312.)  What  i*  an  accidental  image  ?  Illustrate. 
What  is  the  cause  of  accidental  images  ? 


DECOMPOSITION     OF     LIGHT     AND     COLOK.  321 

wafer.  Thus,  if  the  wafer  is  red.  the  image  will  be  green;  if  the 
wafer  is  orange,  the  image  will  be  blue,  and  so  on.  These  images 
are  called  accidental. 

If  the  setting  sun.  which  is  red,  be  viewed  for  some  time,  and  then 
the  eyes  be  directed  to  a  white  wall,  a  green  imago  of  the  sun  will 
l»e  seen,  which  will  last  for  some  instants,  when  a  red  image  will 
nppear;  a  second  green  image  succeeds  it.  and  so  on  till  the  effect 
.•ntirely  ceases. 

It1  \ve  look  for  some  time  at  a  colored  object  on  a  white  ground, 
we  shall  finally  observe  the  object  surrounded  by  a  fringe,  whose 
color  is  complementary  to  that  of  the  body;  thus,  if  a  red  wafer  be 
placed  upon  a  sheet  of  white  paper,  the  fringe  will  be  green.  Such 
fringes  are  called  accidental. 

Shadows  cast  upon  a  wall  by  the  rising  or  setting  sun,  are  tinged 
green,  the  tint  of  the  sun  being  red  at  that  time. 

If  we  examine  several  pieces  of  cloth  of  the  same  color,  the  eye 
becomes  weaned,  and  in  consequence  of  the  accidental  comple 
mentary  color  being  formed,  the  last  pieces  examined  appear  of  a 
different  shade  from  those  first  viewed. 

Many  of  the  phenomena  of  color  may  he  explained  by 
the  principle  of  interference  of  light. 

If  a  molecule  of  ether  be  acted  on  by  a  wave  of  light,  it  will  take 
up  a  vibratory  motion,  at  right  angles  to  the  direction  of  propaga 
tion.  If  it  be  acted  on  simultaneously  by  two  waves,  i'.s  motion 
will  be  the  resultant  of  the  motions  it  would  receive  from  each  acting 
separately.  If,  therefore,  the  wnves  are  in  the.  same  phase  (Art.  147), 
the  molecule  will  have  its  amplitude  of  vibration  doubled;  if  they 
are  in  opposite  phases,  they  will  neutralize  each  other  and  the  mole 
cule  will  remain  at  rest.  This  action  of  one  system  of  waves  on 
another  is  called  interference. 

The  brilliant  colors  of  a  soap  bubble  are  due  to  the  interference  of 
the  two  sets  of  rays  that  are  reflected  from  the  outer  and  inner  sur 
faces  of  the  film  that  constitutes  the  bubble. 

The  colors  of  thin  plates,  like  the  film  of  oil  oh  water,  the  splendid 
colors  of  the  skimmings  of  melted  lead,  the  iridescent  displays  of 
fractured  crystals,  and  the  like,  are  all  due  to  interference  of  light. 

Explain  the  effect  of  gazing  at  ike  setting  sun.    At  a  colored  object  on  a  white 
ground.    Explain  the  phenomenon  of  interference  of  light.     What  are  some  of  the 
color  from  i/i/<rftrf/<ce  / 


POPULAR    PHYSICS 


The  Rainbow. 

313.  The  RAINBOW  is  a  brilliantly  colored  arc,  seen 
after  a  shower  opposite  the  sun. 

The  colors  being  disposed  in  the  same  order  as  in  the 
solar  spectrum,  would  indicate  that  the  bow  is  due  to 
refraction.  Such  is  shown  to  be  the  case.  Figure  209 
shows  the  course  of  the  rays  in  the  formation  of  a  rainbow. 
The  rays  of  light  coming  from  the  sun,  >S,  fall  upon  the 
spherical  rain  drops,  enter  them,  undergoing  refraction,  are 


Fig.  209. 

internally  reflected,  and  then  emerge,  undergoing  a  second 
refraction.  The  result  is  that  the  emergent  light  is  resolved 
into  the  seven  prismatic  colors,  which,  reaching  the  eye  from 
different  drops,  give  rise  to  the  colors  that  are  observed. 

The  ray  which  enters  the  drop,  #,  for  example,  after 
emergence  sends  to  the  eye  a  red  ray,  whilst  that  which 
enters  the  drop  c,  sends  to  the  eye  a  violet-colored  ray; 


(313.)  What  is  a  Ruinbow?    To  what  is  the  bow  due?    Explain  the  course  of 
the  rave,  Fig.  209. 


DECOMPOSITION    OF    LIGHT    AND    COLORS.  323 

intermediate  drops  send  intermediate  colors.     Each  drop 
sends  a  different  color  to  the  eye. 

Analysis  shows  that  it  is  only  at  certain  angles  that  the  refracted 
rays  emerge  with  sufficient  intensity  to  affect  the  eye  with  color. 
Hence  it  is,  that  the  colored  drops  are  arranged  symmetrically  about 
a  line  drawn  through  the  sun  and  the  eye  of  the  observer.  The 
centre  of  the  bow  is  in  this  line;  hence,  as  the  sun  declines  towards 
the  horizon,  the  bow  rises,  and  at  sunset  it  becomes  a  semicircle. 
In  looking  down  into  spray  with  the  back  turned  towards  the  sun, 
a  complete  circular  bow  may  be  seen. 

The  bow  that  we  have  described  is  called  the  primary 
bow,  and  the  colors  in  it  are  arranged  in  the  order  of  the 
prismatic  colors,  the  red  being  on  the  outside. 

Another  bow  is  generally  seen,  concentric  with  the  primary 
bow,  which  is  called  the  secondary  bow.  This  bow  is  formed 
by  light  which  enters  the  drops  is  first  refracted,  then 
twice  internally  reflected,  and  then  emerges,  being  again 
refracted.  The  result  of  this  deviation  is  a  bow  similar  to 
the  first,  but  having  its  colors  arranged  in  a  reverse  order, 
the  red  being  on  the  inside. 

The  inversion  of  colors  arises  from  the  additional  reflection  that 
the  light  experiences.  It  is  observed  that  the  colors  of  the  secondary 
bow  are  not  so  brilliant  as  in  the  primary  ;  this  is  due  to  the  loss 
of  a  portion  of  light,  which  passes  out  of  the  drop  at  each  incidence- 

From  the  nature  of  the  rainbow,  and  the  principle  of  its  formation' 
it  is  plain  that  every  observer  sees  a  different  bow. 

Chromatic  Aberration. 

314.  The  light  that  falls  on  a  lens  is  decomposed  into 
colored  rays  of  different  degrees  of  refrangibility.  These 


How  is  the  bow  formed  ?  Where  is  its  centre  ?  Why  does  the  "bow  enlarge  as 
the  sun  declines  ?  How  may  a  complete  circular  bow  be  seen  ?  "What  is  a  primary 
how?  A  secondary  bow ?  How  is  it  formed  ?  Why  are  the  colors  in  the  secondary 
bow  reversed  in  or^er  ?  flow  do  the  colors  in  the  two  lows  compare  in  brilliancy  ? 
Does  each  observer  see  the  same  bowt  Why  notf  (314.)  What  is  chromatic 
aberration  ? 

14* 


324 


POPULAI:    PHYSICS. 


Pis.  210. 


rays  are  brought  to  different  foci  along  the  axis,  giving  risG 
to  a  multitude  of  partial  images  of  different  colorSj  which 
by  superposition  produce  a  single  image  slightly  indistinct, 
and  fringed  with  all  the  colors  of  the  spectrum.  This  scat 
tering  of  the  colored  rays  to  different  foci,  is  called  chro 
matic  aberration. 

Fig.  210  shows  the  phenomenon  of  chromatic  aberration. 
The  red  rays  being 
less  deviated  than  the 
others,  are  brought 
to  a  focus  beyond 
them  at.r,  whilst  the 
violet  rays  being 
more  refrangible 
than  the  others,  are  brought  to  a  focus  within  them  at  v. 
Bf  tween  v  and  r,  the  intermediate  colors  are  also  brought 
to  foci. 

Achromatic  Combinations. 

315.  An  ACHROMATIC  COMBINATION  consists  of  two  or 
more  lenses  of  different  kinds  of  glass,  so  constructed  as  to 
neutralize  the  effect  of  dispersion. 

The  combination  usually  consists  of  two  lenses 
lens  made  of  crown-glass,  and  a  concave  lens 
made  of  flint-glass,  as  shown  in  Fig.  211. 
Flmt-gku'S  disperses  light  more  than  crown 
glass.  The  combination,  having  its  thickest 
part  at  the  middle,  is  convergent.  The  dis 
persion  of  the  rays  by  one  of  the  lenses  is 
exactly  neutralized  by  a  dispersion  of  them  in 
in  opposite  way,  so  that  the  image  is  nearly 
colorless. 

Such  combinations  of  lenses  are  called  achro 
matic,  and  are  the  ones  used  in  the  construction  of  telescopes. 


a  convex 


Fig.  5211. 


Illustrate.    (  315.)  What  is  an  Achromatic 
are  nsually  combined  ?    Explain  their  notion. 


mibination?    Illustrate.   What  lenses 


OPTICAL     INSTRUMENTS.  325 

V. — THEORY     AND      CONSTRUCTION      OF      OPTICAL      INSTRUMENTS. 

Optical  Instruments. 

316.  The  properties  of  mirrors  and  lenses  have  led  to 
the  construction  of  a  great  variety  of  instruments,  which  by 
increasing  the  limits  of  vision,   have  opened  to  our  senses 
two  new  worlds,  that  had  else  remained  unknown  to  us,  the 
one  on  account  of  its  minuteness,  and  the  other  on  account 
of  its  immensity. 

Of  the  optical  instruments,  the  most  useful  and  interesting 
are  microscopes,  so  called  because  us?d  to  investigate  minute 
objects,  and  telescopes,  sr>  called  because  they  are  employed 
to  examine  distant  objects. 

Besides  these  a  great  variety  of  other  instruments  have 
been  devised,  such  as  the  magic  lantern,  the  phantasma 
goria,  the  solar  microscope,  the  camera  obscura,  and  the 
stereoscope. 

Telescopes. 

317.  A  TELESCOPE  is  an  optical  instrument  for  viewing 
objects  at  a  distance. 

Telescopes  may  be  divided  into  two  classes,  refracting 
telescopes,  and  reflecting  telescopes. 

In  the  first  class  a  lens,  called  the  object  lens,  is  employed 
to  form  an  image  ;  in  the  second  class  a  mirror  or  speculum 
is  employed  for  the  same  purpose ;  in  both  the  image 
formed  is  viewed  by  a  lens,  or  combination  of  lenses,  called 
the  eye-piece.  The  manner  of  arranging  these  component 
parts,  together  with  tho  nature  of  the  auxiliary  pieces  em 
ployed,  determines  the  particular  kind  of  telescope. 


(316)  What  are  some  of  the  most  useful  optical  instruments?  Mention  some 
other  instruments.  (317.)  What  is  a  Telescope?  How  irinny  classes  of  telescopes 
arc  there?  What  is  tho  difference  between  the  two  classes?  What  determines  the 
kin'l  of  telescope? 


326 


POPULAR     PHYSICS. 


A  great  variety  of  devices  have  been  employed  to  obviate  the 
defects  of  spherical  and  chromatic  aberration,  and  at  the  same  time 
to  obtain  a  sufficiency  of  illumination  to  render  vision  distinct. 
Hence  the  variety  of  telescopes  is  very  great.  Only  a  few  of  the 
most  important  will  be  described  in  these  pages. 

rJhe   Galilean  Telescope. 

318.  The  GALILEAN  TELESCOPE,  named  from  its  illustii 
ous  discoverer,  GALILEO,  consists  essentially  of  a  convex  object- 
glass^  which  collects  the  rays  from  an  object,  and  a  concave 
eye-piece,  by  meanc  of  which  the  rays  from  each  point  of 
the  object  are  rendered  parallel,  and  capable  of  producing 
distinct  vision. 

Fig.  212  shows  the  course  of  the  rays  in  the  Galilean 
telescope.  Pencils  of  rays  from  points  of  the  object,  AI2, 
falling  upon  the  object  lens,  0,  are  converged  by  it,  and  tend 
to  form  an  image  beyond  the  eye-piece,  o.  The  concave 
eye-piece  is  placed  so  as  to  intercept  the  rays  coming  from 


the  object-glass,  being  at  a  distance  from  the  image  equal 
to  its  principal  focal  distance.  In  consequence  of  this 
arrangement,  the  pencil  of  light  coming  from  A,  is  converged 
by  the  object-glass,  and  falling  upon  the  eye-piece,  is  di= 
verged  and  refracted  so  as  to  appear  to  the  eye  to  come 
from  a.  In  like  manner  the  pencil  from  J5,  appears  to  the 
eye  to  come  from  b. 

What  are  the  specidl  objects  *o  &«  attained  in  making  a  telescope?  (318.) 
What  is  a  Galilean  Telescope  ?  Describe  it.  Explain  the  course  of  the  rays  in  n 
Galilean  telescop'e. 


OPTICAL    INSTRUMENTS.  327 

The  image  is  erect  and  virtual,  and  because  the  visual  angle  under 
which  the  image  is  seen,  is  greater  than  that  under  which  the  object 
would  be  seen  without  the  telescope,  it  appears  magnified. 

Opera-glasses  are  simply  Galilean  telescopes.  They  possess  the 
advantage  of  showing  objects  in  their  proper  position,  of  being  short 
and  portable,  and  of  being  well  illuminated. 

The  Galilean  telescope  is  not  adapted  to  astronomical  observation, 
because  the  image  formed  is  virtual:  nevertheless  it  was  with  such 
an  instrument  that  GALILEO  discovered  the  satellites  of  Jupiter. 

The  Astronomical  Telescope. 

319.  The  ASTRONOMICAL  TELESCOPE  consists  essentially 
of  two  convex  lenses,  the  one,  o,  being  the  object-lens,  and 
the  other,  0,  the  eye-piece.  The  object-glass  forms  an  in 
verted  image  of  the  object,  which  is  viewed  by  the  eye 
piece. 


Fig.  213. 

Fig.  213  represents  the  course  of  the  rays  in  this  instru 
ment.  A  pencil  of  rays  coming  from  A,  is  converged  by  o, 
to  a  focus  «,  whilst  a  pencil  from  B,  is  brought  to  the  focus 
b.  In  this  manner  the  lens  o,  forms  an  image,  ab,  of  an 
object,  AB,  which  image  is  real  and  inverted.  The  eye 
piece,  0,  is  placed  at  a  distance  from  ab  equal  to  its  prin 
cipal  focal  distance.  The  pencil  coming  from  the  points, 
a  and  &,  of  the  image,  are  refracted  so  as  to  appear  to  come 
from  the  points,  c  and  d.  The  visual  angle,  that  is,  the 


H(no  is  the  image?    Give  an  example  of  a  Galilean  telescope.     T)  '  ei 
tage#?    T/t  the   Galilean  telescope  adapted  to  astronomical  purpoaen  ?    (3l9-) 
What  is  the  Astronomical  Telescope  ?     Explain  the  course  of  tho  rays  in  it. 


328 


POPULAR    PHYSICS. 


angle  formed  by  the  extreme  rays  which  enter  the  eve,  is 
greater  than  it  would  be  in  viewing  the  object  without  the 
telescope,  and  consequently  the  o!  ject  appears  to  be  magni 
fied. 

In  tins,  as  in  all  other  telescopes,  the  eye-piece  is  c?,pable  of  being 
]  ushed  in,  or  drawn  out,  to  enable  the  observer  to  accommodate  it  to 
r  ^r  as  well  us  distant  objects. 

The   object-glass   is   made   as   large   as    possible,   and   should   be 


//'»>/•  ia  thf  eye-pi  fee  crljnxle-1  ? 


OPTICAL    INSTRUMENTS.  329 

achromatic  (Art.  315).  The  eye-glass  is  made  quite  convex,  so  as 
to  magnify  ttie  image  formed  by  the  object-glass. 

Fig.  214  represents  an  astronomical  telescope  mounted  for  use. 
It  rests  upon  a  cast-iron  stand,  \vitli  three  feet,  called  a  tripod.  The 
tripod  supports  a  vertical  axis,  capable  of  turning  around  in  its  sup 
ports;  the  telescope  is  attached  to  the  top  of  this  axis  by  a  hinge 
joint.  These  arrangements  enable  the  observer  to  direct  the  telescope 
to  any  point  of  the  heavens.  The  telescope  may  be  raised  or  de 
pressed  by  means  of  a  rack,  worked  by  toothed  wheels,  set  in  motion 
by  a  crank,  as  shown  at  the  bottom  of  the  figure. 

A  smaller  telescope  with  a  larger  field  of  view  is  attached  to  it,  to 
aid  the  observer  in  fixing  the  instrument  on  any  object.  This  teles 
cope  is  called  the  seeker. 

The  Terrestrial   Telescope. 

3*2O.  The  TERRESTRIAL  TELESCOPE  differs  from  the  as 
tronomical  telescope,  in  having  two  additional  lenses,  which 
together  constitute  what  is  called  sin  erecting  piece.  The 
object  of  the  erecting  piece  is  to  invert  the  image  formed 
by  the  object-lens,  so  that  objects  may  appear  erect  when 
viewed  through  the  telescope. 


Fig.  215  shows  the  course  of  the  rays  in  a  terrestrial 
telescope.  AJB  is  the  object,  o  is  the  object-lens,  m  and  n, 
two  convex  lenses,  constitute  the  erecting  piece,  and  0  is 
the  eye-piece. 

The  erecting  piece  is  so  placed  that  the  distance  of  the 
image,  I,  shall  be  at  a  distance  from  m,  equal  to  its  prin 
cipal  focal  distance. 

WJiat  is  said  of  the  object-glass  and  of  the  eye-piece?  (  S2O.)  In  what  respect 
does  the  Terrestrial  differ  from  the  Astronomical  Telescope  ?  What  is  the  object  of 
the  erecting  piece  ? 


330  POPULAK    PHYSICS. 

A  pencil  of  rays  from  A,  falling  upon  the  object-lens,  is 
converged  to  a  focus  at  the  lower  end  of  the  image,  7";  the 
pencil  proceeding  from  I,  is  converted  into  a  beam  by  the 
lens,  m,  directed  obliquely  upwards,  which  beam  is  con 
verged  to  a  focus  at  i.  In  this  manner  an  erect  image,  i,  is 
formed,  which  is  then  viewed  by  the  eye-piece,  O.  The 
eye-piece  refracts  the  pencils  coming  from  the  image  i.  so 
as  to  make  them  appear  to  come  from  ab. 

The  angle,  under  which  ab  is  seen,  is  the  visual  angle, 
and  being  greater  than  the  angle  under  which  AB  would 
be  seen  without  the  telescope,  the  object  is  magnified. 

The  number  of  times  which  the  visual  angle  of  the  image 
contains  the  visual  angle  of  the  object,  is  the  magnifying 
power  of  the  telescope. 

The  terrestrial  telescope  is  used  at  sea  and  on  land  for  viewing 
objects  at  a  distance.  It  may.  for  convenience,  be  mounted  in  the 
same  way  as  the  astronomiflal  telescope  shown  in  Fig.  214. 

Reflecting  Telescopes. 

321.  A  REFLECTING  TELESCOPE  is   one   in  which  "the 
image  of  a  distant  object  is  formed  by  means  of  a  reflector 
or  speculum,  which  image  is  then  viewed  by  an  eye-piece. 
The  eye-piece  is  either  a  single  lens  or  a  combination   of 
lenses. 

One  of  the  first  telescopes  of  this  description  was  con 
structed  by  NEWTON,  and  this  is  the  only  one  of  the  kind 
which  we  shall  describe  in  detail. 

Newtonian  Telescope. 

322.  Fig.  216  shows  a  NEWTONIAN  TELESCOPE,  as  con. 
structed  by  M.  FKOMENT,  of  Paris,  with  improvements  in 
troduced  by  that  distinguished  physicist. 

Describe  the  course  of  the  rays  in  the  terrestrial  telescope.  What  is  the  magnifying 
power?  What  is  the  use  of  the,  terrestrial  telescope  ?  (  321.)  What  is  a  reflecting 
telescope?  (  322,)  Describe  the  Newtonian  Telescope, 


OPTICAL     INSTRUMENTS. 


331 


Fig.  216. 

Fig.  217  shows  the  same  telescope  in  section,  and  inni 
cates  the  course  of  the  rays  of  light. 

M  is  a  concave  mirror  placed  at  the  bottom  of  a  long 
tube.  This  reflector  tends  to  form  a  small  image  of  an 
object  at  the  other  end  of  the  tube.  But  before  the  rays 
reach  the  image,  they  are  intercepted  by  a  prism  of  glass, 
mn,  so  arranged  that  the  rays  enter  its  first  face  without 


POPULAR     PHYSICS. 


deviation,  and  strike  its  second  face  so  as  to  be  totally  re 
flected,  which  causes  the  image  to  be  formed  at  ab.  The 
image  thus  formed  is  viewed  by  an  eye-piece  through  the 


side  of  the  telescope.  The  eye-piece  in  this  telescope  is 
made  of  two  plano-convex  lenses,  as  shown  in  the  figure, 
the  combined  eifect  of  which  is  to  cause  the  image  to  appear 
in  the  position  BA,  giving  a  great  power  to  the  telescope. 

Fig.  216  shows  the  manner  of  viewing  the  image.  It  also  shows 
a  small  seeker  attached  to  the  tube  of  the  main  instrument,  which  is 
used  in  directing  the  telescope  to  any  required  object. 

Herschel's  Telescope. 

323.  SIR  WILLIAM  HERSCHEL,  of  London,  modified  the 
Newtonian  telescope  by  inclining  the  mirror,  M,  so  as  to 
throw  the  image  to  one  side  of  the  tube,  where  it  could  be 
viewed  by  a  magnifying  eye-piece,  the  observer's  back  be 
ing  turned  towards  the  object. 

The  large  telescope  made  by  this  eminent  astronomer  was  forty 
feet  in  length,  and  the  speculum  had  a  diameter  of  about  five  feet. 
It  was  with  this  gigantic  instrument  that  he  made  some  of  his  most 
brilliant  discoveries. 

Lord  Ross's  Telescope. 

324.  LORD  Ross,  of  Ireland,  has  recently  constructed  a  reflect 


Explain  Fig.  216.  (323.)  What  modification  did  HEUSCIIET.  make  in  tlic  New 
tonian  U-lcscope?  Describe  HKKSCIIKI/S  telescope.  (334.)  Describe  LOUD  Koss's 
telescope. 


OPTICAL     INSTRUMENTS. 


333 


ing  telescope  still  larger  than  HERSCHEL'S.  The  tube  is  56  feet  in 
length,  and  the  diameter  of  the  refleetor  is  more  than  6  i'eet.  The 
speculum  weighs  over  4  tons,  and  the  entire  instrument  more  than  18 
Ions.  This  telescope  is  supported  by  two  walls  of  masonry  48  feet 
high.  72  feet  long,  and  24  feet  distant  from  each  other.  The  in 
strument  is  said  to  have  cost  the  owner  $(50. 000. 

•  Microscopes. 

325.  A  MICROSCOPE  is  a  modification  of  the  telescope, 
for  viewing  near  objects. 

Microscopes,  like  telescopes,  may  be  composed  of  a  combination  of 
lenses  alone,  or  they  may  be  composed  of  a  combination  of  reflectors 
and  lenses.  Reflecting  microscopes  are  but  little  used.  We  shall 
only  describe  the  refracting  microscope,  of  which  there  are  two  kinds, 
the  simple  and  the  compound. 

The   Simple  Microscope. 

326.  The  SIMPLE  MICROSCOPE  consists  of  a  double  con 
vex  lens  of  short 

focal  distance.  It 
is  usually  set  in  a 
frame  of  metal  or 
of  horn,  and  held 
in  the  hand. 
Fi«\  218  shows 

£5 

the  manner  of 
using  it.  It  is 
held  at  a  distance 
from  the  object 
to  be  viewred,  a 
little  less  than  its 
principal  focal  dis 
tance.  In  this 
case,  each  pencil  of  light  falling  upon  it  will  be  deviated  so 

(325.)  *Wh<it  is  a  Microscope?      Hoio  may  a  microscope    be,    constructed? 
(326)  What  is  a  Simple  Microscope?    Explain   Fig    218. 


Fig.  218. 


>o±  POPULAR     PHYSICS. 

as  to  form  a  beam,  whose  axis  passes   through    the  point 
from  which  the  pencil  proceeds,  and  the  optical  centre. 

The  object  uppea/s  of  the  same  size  that  it  would  if  the  eye  were 
placed  at  the  optical  centre  of  the  lens.  Since  the  least  limit  of 
distinct  vision  is  about  eight  inches,  it  follows  that  a  single  micro 
scope  whose  focal  distance  is  one  inch,  \vould  magnify  an  object 


Fie.  219. 


How  is  the  magnifying pmcer  determined  ? 


OPTICAL     INSTRUMENTS. 


eight  times.     If  the   principal    length  were  only  one  quarter  of  an 
inch,  it  would  magnify  thirty-two  times. 

The   Compound  Microscope. 

327.  The  COMPOUND  MICROSCOPE  consists  essentially  of 
a  double  convex  lens  called  the  object-lens,  and  a  second 
double  convex  lens  called  the  eye-piece. 

Fig.  219  represents  a  compound  microscope  and  the 
method  of  using  it.  Fig.  220  shows  the  same  instrument 
in  section,  and  makes  known 
the  course  of  the  rays.  The 
letters  correspond  to  the  same 
parts  in  both  diagrams. 

The  object  to  be  observed 
is  placed  at  «,  between  two 
plates  of  glass  upon  a  support. 
Over  this  is  a  tube,  OAo,  in 
which  are  disposed  the  two 
lenses,  the  object-lens,  o,  be 
ing  at  its  lower,  and  the  eye 
piece,  0,  at  its  upper  ex 
tremity.  The  object,  a,  be 
ing  placed  a  little  beyond  the 
principal  focus  of  the  object- 
glass,  this  lens  produces  a  real 
image,  be,  which  is  inverted. 
The  object-glass,  0,  is  so 
placed  that  its  principal  focus 
is  a  little  beyond  the  image, 
be.  This  lens  then  acts  as  a 
simple  microscope,  and  mag 
nifies  the  image  as  though  it 
were  at  BC. 


(327-)  "What  is  a  Compound  Microscope?    Explain  its    construction,  and  the 
method  of  usina  it. 


330  POPULAR     PHYSICS. 

The  magnifying  power  depends  upon  the  object-lens.  This  power 
is  increased  by  combining  two  or  three  lense?.  as  shown  at.  //,  on 
the  right  of  Fig.  220.  A  second  lens  is  often  added  to  the  eye-piece, 
as  shown  in  the  Newtonian  telescope,  Fig.  217.  for  the  purpose  of 
remedying  the  defect  arising  from  spherical  aberration.  Moreover, 
all  of  the  lenses  are  made  achromatic. 

Microscopes  of  this  kind  are  constructed  whose  magnifying  power 
is  1800  ;  but  what  is  gained  in  power  is  often  lost  in  distinctness.  A 
good  magnifying  power  is  600  in  length  and  breadth,  which  gives 
360,000  in  surface. 

The  object,  when  transparent,  is  illuminated  by  a  mirror,  M. 
which  concentrates  the  light  upon  it.  When  the  object  is  opaque,  it 
is  illuminated  by  a  lens,  L,  which  concentrates  the  rays  upon  it. 

The  microscope  is  used  in  the  study  of  botany  to  discover  the  laws 
of  the  vegetable  world  j  in  entomology  to  study  the  habits  of  minute 
insects:  in  anatomy  and  medicine  to  study  the  laws  of  animal  physi 
ology  •  in  the  arts,  to  discover  the  composition  of  mixtures  ;  in  com 
merce  to  detect  the  nature  of  stuffs,  and  so  on.  Its  use  is  almost 
universal,  either  as  an  instrument  of  research  or  of  curiosity. 

The  Magic  Lantern. 

328.  The  MAGIC  LANTERN  is  an  apparatus  for  forming 
upon  a  screen  enlarged  images  of  objects  painted  on  glass. 
It  was  invented  about  two  hundred  years  ago,  by  Father 
KIRCHEB,  a  German  Jesuit. 

Fig.  221  represents  a  magic  lantern  in  use,  whilst  a  sec 
tion  of  the  same  instrument  is  shown  in  Fig.  222. 

It  is  composed  of  a  box,  in  which  a  lamp  is  placed  before 
a  reflector,  M ;  the  light  is  reflected  upon  a  lens,  L,  and  is 
converged  so  as  to  illuminate  strongly  the  plate  of  glass,  ab, 
upon  which  the  picture  is  painted.  Finally,  a  combination 
of  two  lenses,  m,  acting  as  a  single  convex  lens,  is  placed  so 


Upon  what  does  the  magnifying  pmcer  depend  ?  Why  i«  a  second  lens  added 
to  the  eye-piece?  ffow  great  may  the  magnifying  power  be  made?  How  in  the 
object  illuminated  ?  What  are  some  of  the  uses  of  the  microscope?  (328-)  What 
is  a  Magic  Lantern?  By  -whom  invented?  Describe  the  construction  and  method  of 
using  the  magic  lantera. 


OPTICAL    INSTRUMENTS. 


337 


Fte.  221. 


that  the  plate,  ab,  shall  bo  a  littlo  beyond  its  principal  focus. 
At  this  distance  the  lenses  produce  (as  shown  in  Fig.  196) 
a  magnified  and  inverted  imago  of  the  picture  painted  on 


338  POPULAR     PHYSICS. 

the  glass.     The  picture  on  the  glass  should  be  inverted,  in 
order  that  its  image  may  appear  erect. 

The  image  on  the  screen  will  be  the  more  magnified,  as  the  plate, 
ab.  approaches  the  principal  focus  of  the  compound  lens  m.  It  will 
also  be  the  more  magnified  as  the  compound  lens  increases  in  power. 

The  Phantasmagoria. 

329.  The  PHANTASMAGORIA  differs  from  the  magic  lan 
tern  only  in  having  an  arrangement  by  which  the  size  of  the 
image  on  the  screen  may  be  increased  or  diminished  at 
pleasure. 

The  Polyrama  and  Dissolving  Views. 

330.  The  POLYRAMA  consists  of  a  double  magic  lantern, 
with  two  cut-off  screens.     DISSOLVING  VIEWS  are  obtained 
by  using  both  lanterns.     Thus,  if  a  picture  of  a  daylight 
scene  be  painted  on  one  of  the  slides,  and  of  the  same 
scene  by  moonlight  be  painted  on  the  other,  the  first  picture 
is  thrown  upon  the  screen,  strongly  illuminated,  the  other 
one  being  entirely  excluded  by  a  screen  that  cuts  off  the 
second  lens.     By  an  arrangement  operated  by  the  exhibitor, 
the  light  is  gradually  cut  off  from  the  first  picture  and  ad 
mitted  upon  the  second,  the  first  fading  away  insensibly, 
whilst  the  second  as  gradually  grows  brighter.      In  this 
way  all  the  effects  intermediate  between  full  daylight  and 
full  moonlight  may  be  obtained  in  succession. 

A  volcano,  calm,  and  only  surmounted  by  a  light  cloud  of  smoke, 
may  be  followed  by  a  picture  of  the  same  volcano  sending  forth 
volumes  of  flame  and  smoke.  A  storm  may  be  made  to  succeed  a 
smiling  landscape,  and  so  on;  the  illusion  is  complete. 

The  Photo-Electric  Microscope. 

331.  The  PHOTO-ELECTRIC  MICROSCOPE  is  constructed 
on  the  same  optical  principles  as  the  magic  lantern,  except 

(32Q.)  How  does  the  Phantasmagoria  differ  from  the  Magic  Lantern  ?  (330.) 
What  is  the  Polyrama  ?  Explain  the  method  of  producing  the  Pissolving  Views. 
Illustrate.  (331.)  What  is  the  Photo-Electric  Microscope  f 


OPTICAL    INSTRUMENTS. 


that  the  light  employed  is  obtained  by  passing  an  electric 
current  between  two  charcoal  points.  The  pictures  on  the 
shades  are  also  made  smaller  than  in  the  'magic  lantern, 
which  requires  a  greater  illumination. 


Fig.  224  represents  in  detail  the  arrangement  of  this 
instrument.  At  the  foot  of  the  apparatus  is  a  battery  for 
generating  electricity,  which  will  be  described  hereafter. 
The  electricity  is  conveyed  to  the  charcoal  points  in  the 
box,  .7?,  by  means  of  two  copper  wires,  cne  going  to  the 


Explain  the  arrangement  of  parts. 


340  POPULAR     PHYSICS. 

upper,  and  the  other  to  the  lower  point.  The  points  being 
slightly  separated,  the  circuit  is  completed  only  by  the 
electricity  passing  across  the  interval,  which  gives  rise  to  a 
light  of  extreme  brilliancy. 

In  the  figure,  I  represents  a  parabolic  reflector  for  con 
centrating  the  light  upon  the  slide,  X,  through  a  lens,  C. 
D  is  a  lens  which  forms  a  magnified  image  of  the  minute 
object  on  a  screen.  The  tube  in  which  the  lens,  D,  is 
placed,  may  be  drawn  out  or  pushed  in  to  vary  the  magni 
fying  power  of  the  apparatus. 

The  magnifying  power  of  this  instrument  may  be  made  extremely 
great,  and  by  suitable  management  it  serves  to  show  to  a  large  com 
pany  the  wonders  of  the  microscopic  world. 

One  of  the  most  remarkable  experiments  made  with  it.  is  to  show 
the  circulation  of  the  blood.  Instead  of  a  picture  on  the  slide,  let 
the  tail  of  a  tadpole  be  placed  between  two  plates  of  glass  and  in 
troduced.  There  will  appear  upon  the  screen  what  seems  an  illumi 
nated  map,  all  of  whose  streams  flow  wiih  a  rapid  current.  It  is 
but  the  blood  circulating  with  great  velocity  through  the  arteries 
and  veins. 

The  phenomena  of  crystallization  are  exceedingly  beautiful  when 
seen  by  this  microscope.  If  a  drop  of  a  solution  of  sal  ammoniac, 
for  example,  be  poured  upon  a  plate  of  glass,  and  then  introduced 
into  the  instrument,  the  heat  will  cause  the  water  to  evaporate,  pro 
ducing  one  of  the  most  beautiful  examples  of  crystallization  that  can 
be  exhibited. 

The  minute  animalculse  of  solutions  and  stagnant  water  can  be 
shown  by  this  microscope. 

When  the  light  of  the  sun  is  used  instead  of  the  electric  light,  the 
apparatus  is  called  the  solar  microscope. 

The  Diorama. 

332.  The  DIOEAMA  consists  of  two  pictures,  one  on  each 
side  01  a  transparent  muslin  screen,  these  pictures,  as  in 

How  is  the  magnifying  power  varied?  What  are  its  advantage*?  How  is  the 
circulation  of  the  Hood  shown  ?  The  phenomena  of  crystallization  t  Animal- 
dtlce  f  What  is  a  solar  microscope  t  ( 332.)  What  is  the  Diorama  ? 


OPTICAL     INSTRUMENTS.  341 

the  polyrama,  being  different  effects  of  the  same  scene. 
One  of  these  pictures  is  seen  dh'ectly,  and  the  other  by 
transmitted  light,  and  the  illusion  arises  from  the  light  being 


Fig.  225. 

managed  so  as  to  produce  either  of  these  effects  at  pleasure. 
Fig.  225  explains  the  manner  of  exhibiting  this  kind  of 


From  what  does  the  illusion  arise  ? 


342  POPULAR     PHYSICS. 

picture.  The  two  views  are  painted  on  opposite  sides  of  a 
vertical  screen.  The  first  effect  is  painted  upon  the  front 
of  the  screen,  and  is  seen  by  light  that  enters  a  window,  M, 
and  falling  upon  a  movable  mirror,  E,  is  thrown  so  as  to 
illuminate  the  front  of  the  screen.  The  room  behind  the 
screen  being  dark,  no  part  of  the  picture  on  the  other  side 
of  the  screen  is  seen. 

If,  now,  the  mirror  E,  be  lowered  gently,  the  shutters, 
NN,  being  at  the  same  time  slowly  opened,  the  picture  on 
the  front  of  the  screen  will  fade  away,  to  be  replaced  by  that 
on  the  other  side,  now  seen  by  transmitted  light.  When 
the  mirror  is  let  completely  down,  and  the  shutters,  NN,  are 
completely  opened,  the  only  effect  that  will  be  seen  will  be 
that  from  behind. 

The  diorama  was  invented  and  perfected  by  DA  GUERRE,  the 
celebrated  discoverer  of  the  daguerreotype.  Many  of  his  pictures 
of  this  kind  had  a  high  reputation,  among  which  may  be  mentioned 
his  Midnight  Mass,  and  his  Valley  of  Goldeau. 

The   Camera  Obscura. 

333.  The  CAMERA  OBSCURA  is  an  instrument  used  for 
forming  a  clear  picture  of  objects  upon  a  screen  of  ground 
glass  or  paper. 

It  consists,  Fig.  226,  of  a  closed  box  mounted  on  a  stand, 
having  a  small  hole  on  one  side  and  a  screen  for  receiving 
the  image  on  the  opposite  side.  The  hole  may  be  of  any 
dimensions,  if  a  concave  lens  be  placed  in  it  capable  of  filling 
it,  and  of  such  power  as  to  bring  the  rays  to  a  focus  on  the 
opposite  screen. 

Fig.  226  shows  how  the  image  is  formed  in  the  camera 
obscura.  The  pencil  of  rays  coming  from  the  soldier's  cap 
goes  to  form  an  image  at  the  bottom  of  the  box,  whilst  that 
coming  from  his  feet  goes  to  form  an  image  at  the  top  of 

Explain  the  method  of  exhibiting.     Who  invented  the  diorama  T    (333.)  What 
l?\  the  Camera  Obscura  ?    Describe  it.    Explain  the  course  of  the  rays. 


OPTICAL    INSTRUMENTS, 


Fig.  '2-26 

the  screen.  The  image  is  inverted  and  reversed  in  a  hori 
zontal  direction,  but  in  every  other  respect,  including  color, 
it  is  a  perfect  representation  of  the  object  pictured. 

The  camera  obseura  affords  aid  in  sketching  the  outlines  of  a 
landscape  or  building,  but  its  principal  importance  at  present  consists 
in  its  application  to  the  various  branches  of  Photography.  It  may 
also  be  used  as  a  source  of  amusement. 

The  images  formed  by  a  camera  obseura  possess  the  remarkable 
peculiarity  of  being  entirely  independent  of  the  shape  of  the  opening 
in  the  box,  provided  it  be  quite  small.  The  shape  of  the  images  is 
the  same,  whether  the  opening  be  square,  round,  triangular,  or  ob 
long. 

To  show  this,  let  us  consider  the  case  of  a  beam  of  solar  light 
entering  a  dark  room  through  a  hole  in  a  shutter,  Fig.  227.  With 
respect  to  the  sun,  the  hole  in  the  shutter  is  but  a  point,  hence  the 
group  of  rays  which  enter  it  form  'in  reality  a  cone  whose  base  is  the 
sun.  The  prolongation  of  these  rays  into  the  room  makes  up  another 

For  what  is  the  camera  used  ?  What  remarkalle  property  do  tlie  images  pos* 
««*"  ?  Hoic  is  this  illustrated  t 


POPULAE     PHYSICS. 


is.  22S. 


OPTICAL,   IINTSTKUME:NTS. 


345 


cone  similar  in  shape  to  the  first,  and  if  this  cone  be  intercepted  by 
a  screen  perpendicular  to  the  line  joining  the  hole  with  the  centre  of 
the  sun,  the  image  formed  will  be  a  circle.  If  the  rays  are  inter 
cepted  by  an  oblique  plane,  as  in  the  figure,  the  image  is  elliptical, 
but  it  never  takes  the  form  of  the  hole  when  that  is  small. 

In  accordance  with  this  principle,  we  find  the  illuminated  patches 
of  earth  formed  by  light  passing  between  the  leaves  in  a  forest  of  a 
circular  or  elliptical  shape.  This  is  illustrated  in  Fig.  228.  In  an 
eclipse  of  the  sun,  when  the  visible  portion  of  the  sun  is  of  crescent 
shape,  the  patches  of  light  all  assume  the  crescent  form  :  that  is, 


j,l«in.  the  peculiar  rounded  form  ofpatehM  of  light  in  the  shadow  off'  forest 
nt  form  do  they  take  in  an  eclipse  of  the  sun  ? 


346 


POPULAR     PHYSICS, 


they  are  images  of  the  visible  part  of  the  sun.     The  reason  of  this 
curious  phenomenon  is  evident. 

Manner  of  rendering  the  Image  erect. 

334.  The  manner  of  producing  erect  images  of  external 
objects  in  a  camera  obscura,  or  dark  room,  is  shown  in 
Fig.  229.  A  little  above  the  hole  a  plane  mirror  is  so  placed 
as  to  reflect  the  rays  which  enter  it  upon  a  convex  lens 
fixed  at  the  extremity  of  a  tube.  This  reflection  inverts  the 
beam  of  light  and  makes  the  image  erect,  which  may  then 
be  thrown  upon  a  suitable  screen  for  observation. 

Such  images  are  perfect  representations  of  the  external  objects 
which  they  represent,  being  perfectly  faithful,  not  only  in  form  and 
color,  but  in  motion  also.  When  images  of  street  scenes,  with  all 
their  life  and  motion,  are  thus  formed,  they  are  very  striking  as  well 
as  interesting. 


C  334.)  H<rw  are  the  images  mnrle  erect? 


OPTICAL    INSTRUMENTS.  347 


Portable  Camera  for  Artists. 

335.  For  taking  views,  the  camera  obscura  should  be 
light  and  portable.  The  best  form  is  that  shown  in  Fig.  230. 
It  consists  of  a  sort  of  portable  tent  of  black  cloth,  within 
which  is  a  table  for  receiving  the  image,  and  at  the  top  of 
which  is  a  tube  bearing  a  prismatic  lens,  that  produces  the 
combined  effect  of  the  mirror  and  lens,  as  shown  in  Fig.  229. 
The  figure  projected  upon  the  table  may  be  traced  out  with 
a  pencil  on  a  sheet  of  white  paper. 


Fig.  231. 


Fig.  231  shows  the  course  of  the  rays  in  forming  the 
image.  The  rays  coming  from  the  object,  AJ3,  fall  upon 
the  convex  face  of  the  lens  and  are  converged,  and  in  this 
state  they  reach  the  plane  surface,  m,  which  is  inclined  to 
the  horizon.  Being  totally  reflected  from  the  surface,  m, 
they  emerge  through  the  slightly  concave  surface  below, 
arid  go  to  form  an  image,  ab,  on  the  table,  P.  A  sheet  of 
paper  is  spread  on  P,  to  receive  the  image,  and  on  it  the 
outlines  may  be  traced. 

The  Daguerreotype. 

336.     One  of  the   most   important   applications  of  the 
camera  obscura,  is  in  forming  pictures  upon  plates  of  pre- 
035.)  Explain  the  construction  of  the  Portable  Camera  for  Artists.    Explain  tho 
c»ursb  cff  the  rayd     (  830  j  What  is  the  mtfst  important  application  of 


34 S  roruLAK  PHYSICS. 

pared  metal  or  paper,  by  the  actinic  or  chemicvl  action  of 
the  light. 

The  discovery  of  the  daguerreotyping  process,  like  many  other 
discoveries  of  magnitude,  was  preceded  by  many  partially  successful 
efforts.  One  of  the  most  important  of  them  was,  perhaps,  that  of 
TALBOT,  who  succeeded  in  fixing  images  on  prepared  paper  by  means 
of  .solar  tight.  The  main  discovery  is,  however,  due  to  M.  DA 
GUERRE.  who  in  1839  announced  that  he  could,  by  a  process  occu 
pying  but  a  few  minutes,  fix  the  image  of  a  camera  upon  a  metallic 
plate. 

During  the  few  years  that  have  elapsed,  improvements  have  fol 
lowed  each  other  in  rapid  succession,  until  the  process  of  daguerrco- 
typing,  in  all  its  various  branches,  gives  remunerative  employment 
to  thousands.  It  is  not  only  one  of  the  most  interesting  discoveries 
of  modern  times,  but  it  has  become  of  immense  utility. 

Process   of  Daguerre. 

337.  The  process  of  Daguerre  begins  by  receiving  the  image  of 
the  camera  upon  a  proper  plate,  covered  with  a  thin  layer  of  silver, 
•whose  surface  has  been  carefully  polished  and  rendered  sensitive  to 
light.  The  polished  plate  is  rendered  sensitive  by  means  of  iodine. 
Iodine  is  solid  at  ordinary  temperatures,  but  is  easily  converted  into 
vapor  by  a  slight  degree  of  heat.  The  plate  is  held  over  the  vapor 
of  iodine  for  about  two  minutes,  during  which  time  a  thin  layer  of 
the  silver  unites  with  the  iodine,  forming  a  coating  of  iodide  of  silver, 
which  is  exceedingly  sensitive  to  light.  The  plate  thus  prepared  is 
placed  in  the  camera,  so  as  to  receive  the  image  to  be  copied,  and  is 
acted  upon  by  the  rays  forming  the  image.  The  plate  is  next 
exposed  for  a  few  minutes  to  the  vapor  of  mercury.  The  mercury 
unites  with  the  silver  where  it  has  been  acted  upon  by  the  light, 
forming  a  white  amalgam,  giving  the  lights  of  the  picture,  whilst 
the  other  parts  remain  dark. 

This  process  was  imperfect ;  the  plates  required  ten  or  twelve 
minutes'  exposure  to  light,  in  order  to  fix  an  impression,  which 
rendered  the  method  unsuitable  for  portraits;  the  pictures  formed 
were  indistinct  and  easily  effaced,  and  finally,  the  reflected  light 


Girt  f>  sketch  of  the  history  of  the  Daguerreotype.    (337.)  Explain  the  process 


OPTICAL     INSTRUMENTS. 


349 


from  the  plates  diminished  the  distinctness  of  vision.     All  of  these 
defects  were  remedied  by  a  single  man,  M.  FIZEAU 

By  using  bromine  with  iodine  in  preparing  the  plates,  he  rendered 
them  so  sensitive,  that  from  six  to  thirty  seconds  formed  n,  sufficient 
exposure.  He  fixed  the  images  and  prevented  excessive  reflection^ 
by  using  chloride  of  gold  and  hyposulphite  of  soda  with  gentle  heat. 
This  process  not  only  had  the  effects  named,  but  it  also  increased 
the  brightness  of  the  picture.  Since  these,  other  improvements 
have  been  made,  till  at  last  in  skillful  hands  it  has  reached  a  state 
of  great  perfection. 


Fig.  232  represents  the  form   of  camera   used  in  the  proces? 
daguerreotyping.     It   consists    of  a  rectangular  wooden  box,  to 


of 
one 


Explain  the  modifications  O/FIZEAIT.     Explain  the  construction  and  mttfiod  of 
uning  the  camera  for  daguerreotyjiing. 


350  POPULAR     PHYSICS. 

lace  of  which  is  attached  a  tube,  bearing  a  lens,  which  forms  the 
image.  The  opposite  face  of  the  box  consists  of  a  sliding  drawer, 
holding  a  plate  of  ground  glass,  upon  which  the  image  is  thrown, 
and  by  drawing  it  out,  or  sliding  it  in,  the  picture  may  be  rendered 
distinct  upon  the  glass.  When  the  image  is  clearly  defined,  the 
plate  of  glass  is  removed,  and  the  prepared  silver  plate  introduced, 
and  the  process  above  described  is  performd. 

Photography. 

338.  PHOTOGRAPHY  is  the  art  of  fixing  the  picture  of 
the  camera  oil  paper  or  glass. 

There  are  two  kinds  of  photographic  pictures,  positive 
and  negative.  Positive  pictures  are  those  that  have  their 
lights  and  shades  in  their  proper  relative  position  ;  negative 
pictures  are  those  in  which  the  lights  and  shades  are  re 
versed  in  position.  A  negative  picture  taken  on  glass  is 
used  to  produce  a  positive  one  on  paper. 

To  produce  a  negative,  a  plate  of  glass  is  carefully  cleaned  and 
coated  with  a  layer  of  collodion  impregnated  with  iodide  of  potas 
sium  ;  the  plate  is  then  immersed  for  about  a  minute  in  a  bath  of 
nitrate  of  silver,  containing  thirty  grains  of  the  nitrate  to  an  ounce 
of  water.  The  double  decomposition  that  ensues  gives  rise  to  a 
layer  of  iodide  of  silver,  evenly  spread  on  the  plate.  This  opera 
tion  should  be  performed  in  a  dark  room.  The  plate  is  next 
drained,  and  when  nearly  dry,  it  is  inserted  in  a  closed  frame  and 
exposed  to  the  action  of  the  camera.  The  plate  is  then  removed 
to  a  dark  room,  and  the  picture  is  brought  out,  or  developed,  by 
pouring  over  it  a  solution  of  protosulphate  of  iron  or  pyrogallic 
acid.  This  brings  out,  or  develops,  the  invisible  picture  formed 
by  the  action  of  light  on  the  iodide  of  silver.  When  the  pic 
ture  is  sufficiently  brought  out,  water  is  poured  over  the  plate, 
which  stops  the  further  development.  The  parts  not  acted  on  by 
light  still  contain  iodide  of  silver,  which,  if  not  removed,  would  bo 
affected  if  exposed  to  the  action  of  light.  This  is  removed  by  wash 
ing  the  plate  with  hyposulphite  of  soda,  which  dissolves  the  iodide 

(.338.)  What  is  Photography  ?  What  are  positive  and  negative  pictures  ?  How 
is  a  negative  on  glass  produced  ? 


OPTICAL   INSTRUMENTS.  351 

but  does  not  affect  the  picture.  The  plate  thus  prepared  is  dried, 
and  then  coated  with  a  thin  layer  of  transparent  varnish  to  protect  it 
from  injury. 

The  negative  thus  produced  is  used  for  obtaining  positive  prints 
on  paper,  as  follows : — Paper  is  impregnated  with  chloride  of  silver 
by  first  immersing  it  in  a  solution  of  nitrate  of  silver  and  then  in  one 
of  chloride  of  sodium  ;  chloride  of  silver  is  thus  formed  on  the  paper 
by  double  decomposition.  The  negative  is  placed  over  a  sheet  of 
this  prepared  paper  and  exposed  to  the  action  or  sunlight  for  a  cer 
tain  time.  The  chloride  of  silver  is  acted  on  by  the  light  shining 
through  the  negative,  being  most  affected  under  the  light  parts  of  the 
negative  and  least  affected  under  the  dark  parts.  The  copy  thus 
formed  is  a  positive  picture.  In  order  to  fix  it,  the  paper  is  thor 
oughly  washed  in  a  solution  of  hyposulphite  of  sodium,  which  dis 
solves  out  the  unaltered  chloride  of  silver  and  prevents  the  further 
action  of  light  The  picture  is  next  immersed  in  a  bath  of  chloride 
of  gold  to  give  it  the  proper  tone. 

To  obtain  a  positive  picture  on  glass,  prepare  the  plate  as  before. 
After  exposure  to  the  action  of  the  camera,  develop  by  a  solution 
of  protosulphate  of  iron  ;  this  gives  a  negative ;  then  pour  over  the 
plate  a  solution  of  cyanide  of  potassium  ;  this  rapidly  changes  the 
negative  into  a  positive.  This  completed,  the  picture  is  washed, 
dried,  and  a  coating  of  varnish  is  poured  over  its  surface. 

Besides  the  methods  given  above,  there  are  many  others  in  use, 
producing  particular  varieties  of  picture,  but  all  depend  on  the  same 
fundamental  principle,  that  is,  the  extreme  sensitiveness  of  salts  of 
silver  to  the  action  of  light. 

Structure  of  the  Eye. 

339.  The  EYE  is  a  collection  of  refractive  media,  by 
means  of  which  we  are  made  acquainted  with  the  external 
world  through  the  sense  of  sight. 

As  an  optical  instrument  the  eye  is  inimitably  perfect;'  it  has  not 
the  faults  either  of  spherical  or  chromatic  aberration,  and  withal,  it 
possesses  the  remarkable  property  of  self-adaptation  to  great  as  well 
as  small  distances. 

The  shape  of  the  eye  is  spherical,  with  a  slight  protuher- 

Hoiv  are  negatives  on  glass  used  to  obtain  positive  prints  f  How  are  positives  on 
glass  obtained?  (839.)  What  is  the  eye ? 


352  OPTICAL     INSTRUMENTS. 

ance  in  front ;  the  average  diameter  of  the  human  eye  is 
a  little  less  than  nine  tenths  of  an  inch.  Fig.  233  represents 
a  section  of  an  eye,  with  some  of  the  coverings  thrown  back 
so  as  to  show  the  position  of  the  parts. 


fig.  233. 

The  anterior  part  of  the  eye  is  limited  by  a  perfectly 
transparent  membrane,  c,  called  the  cornea.  The  remainder 
of  the  exterior  coating  is  an  opaque  white  membrane,  called 
the  sclerotic  coat.  The  cornea  is  set  in  the  sclerotic  coat, 
a*  a  watch-glass  is  set  in  its  frame. 

Immediately  behind  the  cornea  is  a  transparent  fluid, 
limpid  as  water,  called  the  aqueous  humor.  In  this  floats 
a  circular  curtain,  7u,  attached  by  its  outer  edge  to  the 
sclerotic  coat,  and  having  a  small  circular  opening  at  its 
middle.  The  curtain  is  called  the  em,  and  the  hole  in  its 
centre  is  called  the  pupil.  The  iris  gives  color  to  the  eye, 
being  black,  blue,  gray,  <fcc. ;  it  is  muscular,  and  by  the  con 
traction  and  expansion  of  the  fibres  the  pupil  may  be  en 
larged  or  diminished ;  it  is  through  the  pupil  that  rays  of 
light  enter  the  eye. 

Behind  the  iris  is  a  double  convex  Ions,  o,  called  the 
crystalline  lens  /  it  is  of  the  consistence  of  gristle1,  perfectly 
transparent,  more  curved  behind  than  in  front,  and  is  denser 
towards  its  middle  than  at  the  edges.  This  lens  serves  to 

Its  size  ?     What  is  the  character  and  position  of  the  cornea  ?     Of  the  sclerotic  coat  ? 
Of  the  aqueous  humor  ?    The  iris?    The  pupil  ?     The  Crystalline  lens? 


POPULAR     PHYSICS.  353 

converge  the  rays  to  foci  behind  it.  Immediately  behind 
the  crystalline  lens  is  a  medium  nearly  filling  the  remainder 
of  the  cavity  of  the  eye,  called,  the  vitreous  humor  •  it  is  of 
the  consistence  of  jelly,  and  perfectly  transparent,  permit 
ting  the  rays  to  pass  through  it. 

Immediately  behind  the  vitreous  humor  is  a  thin  white 
xpansion  of  the  optic  nerve,  lining  nearly  all  of  the  sclerotic 
coat ;  this*  is  called  the  retina,  and  is  the  seat  of  vision. 
Behind  the  retina,  and  between  it  and  the  sclerotic  coat,  is 
a  fine  velvety  coating  called  the  choroid  coat,  covered  with 
a  black  pigment,  which  absorbs  the  rays  that  pass  the  retina, 
preventing  internal  reflection.  The  sensation  of  sight  is 
conveyed  to  the  brain  by  the  optic  nerve,  which  goes  to  the 
brain. 

The   Mechanism    of  Vision. 

340.  The  action   of  the  eye  is  similar  to  that  of  the 
camera  obscura,  except  more  perfect ;  the  pupil  corresponds 
to  the  hole  in  the  shutter,  the  crystalline  lens  forms  the 
image,  and  the  retina  is  the  screen  on  which  the  image  falls. 
The  image  formed  is  of  course  inverted,  as  shown  in  Fig. 
233,  but  the  mind  refers  objects  along  the  rays  which  pro 
duce  the  sensation  of  sight,  hence  points  appear  in  their 
proper  position  ;  that  is,  we  see  objects  erect. 

Limit  of  Distinct   Vision.  —  Defects   of  Sight. 

341.  When  an  object  is  placed  very  near  the  eye,  the 
lens  has  not  sufficient  power  to  bring  the  rays  to  foci  on  the 
retina,  and   an  indistinctness  of  vision  is  the  consequence. 
The  least  distance  at  winch  an  object  can  be  seen  distinctly 
is  very   different  in   different  individuals.     It  may,   on   an 
average,  be  put  down  at  six  inches.     Sometimes  this  limit 
is  not  the  same  for  both  eyes  in  the  same  individual. 


The  vitreous  humor?  The  retina?  The  choroid  coat?  The  optic  nerve? 
"(34O.)  Describe  the  mechanism  of  vision.  (241.)  "What  is  the  average  limit  of 
distinct  vision  ? 


oO-i  OPTICAL     INSTRUMENTS. 

When  the  limit  of  distinct  vision  is  much  less  than  six 
inches,  the  individual  is  said  to  be  short-sighted  /  when  it 
is  much  greater  than  six  inches,  he  is  said  to  be  long 
sighted. 

SHORT-SIGHTEDNESS  comes  from  too  great  convexity  of  the 
cornea,  or  crystalline  lens,  or  both.  The  effect  is  to  bring 
tho  rays  to  foci  before  reaching  the  retina,  giving  an  indis 
tinctness  to  vision.  This  defect  is  remedied -by  using 
spectacles  'with  concave  lenses,  which  diverge  the  rays 
before  falling  upon  the  cornea,  and  thus  enable  the  media 
of  the  eye  to  bring  them  to  foci  upon  the  retina.  If  the 
eyes  are  unlike,  the  lenses  should  be  of  different  power. 

LONG-SIGHTEDXESS  is  a  defect  just  the  reverse  of  short 
sightedness.  It  arises  from  too  great  flatness  in  the  cornea, 
or  crystalline  lens,  so  that  rays  of  light  are  brought  to  foci 
behind  the  retina.  This  defect  is  remedied  by  using  spec 
tacles  with  convex  lenses. 

Short-sightedness  is  a  defect  of  youth,  and  is  gradually 
removed  as  the  individual  advances  in  years ;  long-sighted 
ness  is  a  defect  of  advanced  age,  and  once  commenced,  it 
gradually  increases  with  years,  probably  because  the  organs 
which  secrete  the  media  of  the  eye  become  feeble  as  life 
advances. 

The  best  form  of  convex  glasses  for  spectacles  is  the 
meniscus,  0,  Fig.  186,  and  the  best  form  of  concave  glasses 
is  the  concavo-convex,  JR,  Fig.  187.  These  glasses  are  called 
periscopic,  because  they  permit  a  wider  range  of  vision  than 
other  forms  of  lenses. 

Vision  with  two   Eyss. 

342.  An  image  of  every  object  viewed  is  formed  in  each 
eye,  yet  vision  is  not  double,  but  single.  This  is  regarded 

"When  Is  a  person  short-sighted  ?  When  long-sighted  ?  What  is  tho  cause  of  short 
sightedness?  How  is  it  remedied  ?  What  is  the  cause  of  long-sightedness  ?  How 
is  it  remedied?  What  are  periscopic  glasses?  (342-)  How  are  we  enabled  to  see 
clearly  with  two  eyes  ? 


POPULAR    PHYSICS. 


355 


by  some  as  a  matter  of  habit ;  others  refer  it  to  the  fact  that 
each  nervous  filament  coming  from  the  brain  to  the  eye  is 
divided  into  two  parts,  one  going  to  each  eye. 

Simultaneous  vision  with  two  eyes  is  supposed  to  give  us 
the  idea  of  relief,  or  form  of  objects,  a  view  which  receives 
confirmation  from  the  action  of  the  stereoscope. 

The   Stereoscope. 

343.  The  STEREOSCOPE  is  an  apparatus  employed  to  give 
to  flat  pictures  the  appearance  of  relief;  that  is,  the  appear 
ance  of  having  three  dimensions. 

It  was  invented  by  WHEA.TSTONE  and  improved  by  BREWSTER. 
At  the  present  day  it  is  offered  for  sale  in  a  great  variety  of  forms, 
and  constitutes  an  instructive  and  amusing  instrument. 

When  we  look  at  an  object  with 
both  eyes,  each  eye  sees  a  slightly 
different  portion  of  it.  Thus,  if  we 
look  at  a  small  cube,  as  a  die,  for 
example,  first  with  one  eye  and 
then  with  the  other,  the  head  re 
maining  fast,  we  shall  observe  that 
the  perspective  of  the  cube  is  dif 
ferent  in  the  two  cases.  This  will 
be  the  more  apparent  the  nearer 
the  body. 

If  the  cube  has  one  face  directly 
in  front  of  the  observer,  and  the 
right  eye  is  closed,  the  other  eye 
will  see  the  front  face  and  also  the 
left  hand  face,  but  not  the  right ;  if, 
however,  the  left  eye  is  closed,  the  other  eye  will  see  the 
front  face  and  also  the  right  hand  face,  but  not  the  left. 

Whence  do  we  derive  our  notion  of  relief  in  bodies  ?  (343.)  What  is  the  Stereo 
scope  ?  By  whom  invented  f  Explain  the  theory  and  construction  of  the  stereoscope 
in  detail. 


Fig.  234. 


356 


OPTICAL     INSTRUMENTS. 


Hence  we  know  that  the  two  images  formed  by  the  two  eyes 
are  not  absolutely  alike.  It  is  this  difference  of  images  which 
gives  the  idea  of  relief  in  looking  at  a  solid  body. 

If,  now,  we  suppose  two  pictures  to  be  made  of  an  object, 
the  one  as  it  would  appear  to  the  right  eye  and  the  other  as 
it  would  appear  to  the  left  eye,  and  then  look  at  them  with 
both  eyes  through  lenses  that  cause  the  pictures  to  coincide, 
the  impression  is  precisely  the  same  as  though  the  object 
itself  were  before  the  eyes.  The  illusion  is  so  complete, 
that  it  is  almost  impossible  to  believe  that  we  are  simply 
viewing  pictures  on  a  flat  surface. 

Such  is  the  theory  of  the  stereoscope.  Fig.  234  shows  the  course 
of  the  rays  in  this  instrument  as  just  described.  A  represents  a 
picture  of  the  object  as  it  would  be  seen  by  the  right  eye  alone  •  jB, 
a  picture  of  the  same  object  as  it  would  be  seen  by  the  left  eye 
alone  ;  m  and  n  are  lenses  which  deviate  the  rays  so  as  to  make  the 
pictures  appear  to  be  coincident  in  C. 

The  lenses,  m  and  /i,  ought  to  be  perfectly  symmetrical,  and 
BREWSTER  attained  this  result  by  cutting  a  double  convex  lens  in 


Fig.  235. 


Explain  the  course  of  the  rays  in  the  stereoscope. 


POPULAR     PHYSICS.  357 

two,  and  placing  the  right  hand  half  before  the  left  eye,  and  the 
other  half  before  the  right  eye.  The  pictures  must  be  perfectly 
executed,  which  can  be  done  only  by  means  of  the  daguerreotype  or 
photographic  process.  The  pictures  are  made  by  using  two  cameras 
inclined  to  each  other  in  the  proper  angle. 

Fig.  235  represents  two  stereoscopic  pictures  of  FRANKLIN,  taken 
from  a  statue.  We  see  the  left  hand  one  more  in  front,  the  right 
hand  one  more  in  profile.  On  placing  them  in  the  stereoscope  we 
see  a  single  image  in  relief.  This  image  stands  out  in  relief,  pre 
senting  all  the  appearance  of  the  statue  from  which  the  pictures  are 
taken. 

The  best  form  of  the  stereoscope  is  that  of  DUBOSCQ.  The  lenses 
are  large,  and  touch  each  other,  so  that  they  are  adapted  to  eyes  which 
are  at  any  distance  apart,  which  is  not  the  case  in  the  instrument 
shown  in  Fig.  234.  In  that  instrument  the  eyes  must  be  at  a  certain 
distance  apart,  which  does  not  permit  the  same  instrument  to  be 
used  by  both  children  and  adults. 


Explain  BBEWSTEE'S  form. 


CHAPTER  VII. 

MAGNETISM. 
I. GENERAL       PROPERTIES      OF       MAGNETS. 

Definition   of  Magnetism. 

344.  MAGNETISM,  as  a  science,  is  that  branch  of  Physics 
which  treats  of  the  properties  of  magnets,  and  of  their  action 
upon  each  other. 

Magnets. 

345.  A  MAGNET  is  a  body  which  exercises  a  particular 
power  of  attraction  upon  iron  and  a  few  other  metals. 

Magnets  are  either  natural  or  artificial 
Natural  magnets  are  certain  ores  of  iron,  and  are  gener 
ally  known  under  the  name  of  loadstones. 

The  magnet  is  so  called  from  the  town  of  Magnesia,  in  Lydia, 
•where  it  was  first  noticed  by  the  Greeks.  In  its  natural  form  it  con 
sists  of  a  mixture  of  two  oxides  of  iron,  with  a  small  proportion  of 
quartz  and  alumina.  It  is  now  found  in  considerable  quantities  in 
Sweden  and  Norway,  as  well  as  in  many  other  countries. 

The  magnet  possesses  the  remarkable  power,  when  freely 
suspended,  of  directing  itself  towards  a  particular  point  of 
the  horizon,  and  it  is  to  this  property  that  its  importance  is 

(  344.)  "What  is  Magnetism  as  a  science?  What  is  a  Masrnet?  How  many  kinds 
of  magnets  are  there ?  What  are  natural  magnets ?  Whence  the  name?  Wh<>t  i* 
the  constitution  of  a  natural  magntt  t  What  remarkable  property  does  the  magnet 
possess  ? 


GENERAL  PROPERTIES  OF  MAGNETS.         359 

chiefly  due.  It  may  be  suspended  by  a  thread,  or  by  bal 
ancing  it  on  a  pivot.  In  practice  the  latter  method  is  the 
one  most  usually  adopted. 

Artificial  magnets  are  bars  of  tempered  steel,  to  which 
the  property  of  the  natural  magnet  has  been  imparted.  The 
artificial  magnet  is  far  more  valuable  than  the  natural 
magnet,  and  is  generally  used  in  practice. 

S:eel  is  a  mixture  of  iron  with  a  small  quantity  of  carbon,  and 
when  heated  and  then  plunged  into  water,  it  becomes  exceedingly 
hard,  and  capable  of  retaining  the  magnetism  that  may  be  imparted 
to  it. 

Artificial  magnets  for  experiment  are  made  of  oblong  bars,  from 
twelve  to  fifteen  inches  in  length,  as  represented  in  Figs.  245  and 
246.  They  are  sometimes  made  in  the  form  of  a  horse-shoe,  as 
shown  in  Fig.  247.  Sometimes  they  are  made  in  the  form  of  a  thin 
long  needle,  as  shown  in  Fig.  239.  This  is  the  form  in  which  they 
are  constructed  for  pointing  out.  the  direction  of  the  magnetic  me 
ridian,  as  in  compasses.  In  this  form  they  are  also  used  in  many 
magnetic  experiments. 

Magnets  may  be  made  of  soft  iron  or  untempered  steel,  but  they 
do  not  retain  their  magnetism  when  the  exciting  cause  is  removed. 
Such  magnets  are  called  temporary  magnets. 

Distribution   of  Force  in   Magnets. 

346.  The  force  with  which  a  magnet  attracts  iron,  is 
not  the  same  in  all  of  its  parts.  The  attraction  is  strongest 
at  its  extremities,  from  which  it  decreases  towards  its 
middle,  where  it  is  nothing. 

This  may  be  shown  by  plunging  one  end  of  a  magnetized  bar  into 
iron  filings  ;.  on  withdrawing  it,  the  filings  will  be  seen  adhering  to 
it  in  long  filaments,  as  shown  in  Fig.  237. 

If  the  entire  bar  be  rolled  in  the  filings,  it  wrill  be  found  that  they 
adhere  to  both  ends,  but  not  to  the  middle. 


What  is  an  artificial  magnet?  What  is  steel?  Describe  an  artificial  magnet. 
What  are  temporary  magnets?  (346.)  Where  is  the  attraction  strongest?  How 
thownT 


160 


POPULAR     PHYSICS. 


The  two  ends,  where  the  attraction  is  strongest,  are  called 
poles,  and  the  central  part,  where  the  attraction  is  nothing, 
is  called  the  equator,  or  the  neutral  line. 


Fig.  287. 

Every  magnet  has  two  poles  and  one  neutral  line,  whether  th« 
magnet  be  natural  or  artificial.  Sometimes,  besides  the  two  prin 
cipal  poles,  there  are  other  minor  poles,  called  secondary  poles.  In 
artificial  magnets  these  arise  from  inequality  of  temper  in  the  steel 
bars,  or  from  want  of  proper  care  in  magnetizing  them.  We  shall 
suppose  each  magnet  to  have  but  two  poles. 

The  action  of  a  magnet  upon  iron  takes  place  through  intermediate 
bodies.  If  a  magnetized  bar  be  covered  with  a  sheet  of  paper,  and 


What  are  poles  ?    Equator  ?     What  are  secondary  pole*  ?     Their  origin  f    How 
is  it  ahown  that  magnetism  is  exeritd  through  intermediate  lodies  f 


GENERAL    PROPERTIES    OF    MAGNETS. 


361 


then  fine  iron  filings  be  sifted  uniformly  over  the  paper,  they  will  be 
seen  arranging  themselves  in  regular  curves  around  each  pole,  as 
shown  in  Fig.  238.  No  action  is  observed  about  the  neutral  line, 
the  filings  falling  there  as  on  any  other  surface. 


Fig.  238. 

Hypothesis   of  two   Magnetic   Fluids. 

347.  If  we  compare  the  action  of  the  two  poles  upon 
soft  iron,  we  observe  the  same  phenomena  at  both.  It  is 
not  so,  however,  when  we  compare  the  action  of  two  mag 
nets  upon  each  other.  If  to  the  same  pole  of  a  magnetic 
needle,  db,  balanced  on  a  pivot  (Fig.  239),  we  present  in 
succession  the  two  poles  of  a  magnetized  bar,  held  in  the 
hand,  we  observe  the  curious  phenomena,  that  if  the  pole, 
a,  of  the  needle  is  attracted  by  the  pole,  J5,  of  the  bar,  the 
pole,  5,  will  be  repelled  by  it;  if  the  pole,  a,  is  repelled,  the 
pole,  &,  will  be  attracted. 

To  explain  these  phenomena,  it  has  been  supposed  that 
there  are  two  magnetic  fluids,  that  is  to  say,  two  kinds  of 


(347.)  What  is  £ha  action  of  one  magnet  upon  another?    What  is  the  theory 
of  two  fluids? 

16 


3t>2 


POPULAR     PHYSICS. 


subtile  matter  surrounding  the  molecules  of  the  magnet, 
each  fluid  repelling  its  own  kind,  and  attracting  the  other 
kind. 

According  to  this  hypothesis,  a  body  is  magnetized  when 
these  fluids  are  separated  and  driven  to  its  opposite  extrem- 
i  ies.  The  difference  of  the  two  poles  arises  from  the  nature 


Fig.  239. 

of  the  fluids  which  predominate  in  them ;  the  poles  which 
contain  the  same  kind  of  fluid,  repel,  those  which  contain 
opposite  kinds,  attract  each  other.  The  attraction  and  re 
pulsion  are  mutual. 

Another  theory  supposes  but  one  kind  of  magnetic  fluid,  and  ex 
plains  the  phenomena  by  supposing  this  to  exist  in  excess  at  one  pole, 
and  in  defect  at  the  opposite  pole.  Either  theory  explains  the  phe 
nomena,  but  that  of  two  fluids  is  the  most  easily  applied,  and  for 
that  reason,  solely,  it  is  adopted. 

The  earth,  as  we  shall  see  hereafter,  resembles  a  huge  magnet, 
acting  upon  magnetic  needles  in  the  same  way  that  magnetized  bars 

When  is  a  body  magnetized  according  to  this  theory  ?  What  other  theory  is  there  V 
Describe  the  magnetic  action  of  the  earth. 


GENERAL    PROPERTIES     OF    MAGNETS.  363 

do.  Its  magnetic  poles  are  near  the  geographic  poles  of  the  earth, 
and  the  neutral  line  coincides  very  nearly  with  the  equator.  Con 
sequently  the  fluid  which  is  supposed  to  predominate  near  the  north 
pole  of  the  earth  is  called  the  boreal  fluid^  and  that  which  is  sup 
posed  to  predominate  near  the  south  pole  of  the  earth  is  called 
the  austral  fluid. 

Because  dissimilar  poles  attract  and  similar  ones  repel,  it  follows 
that  the  pole  of  a  balanced  magnetic  needle  which  turns  towards  the 
north  must  contain  the  austral  fluid,  whilst  the  one  which  turns 
towards  the  south  must  contain  the  boreal  fluid. 


Laws  of  Attraction  and  Repulsion. 

348.  The   following    laws    have    been     suggested    by 
theory  and  confirmed  by  experiment : 

1.  Magnetic  poles  of  contrary  names  attract,  and  those 
of  the  same  name  repel  each  other. 

2.  The  forces  of  attraction  and  repulsion  both  vary  in 
versely  as  the  square  of  the  distance  between  the  attracting 
an  d  repelling  poles. 

Magnetic   and  Magnetized  Bodies. 

349.  A  MAGNETIC  BODY  is  one  which  contains  the  two 
magnetic  fluids,  but  in  a  state  of  equilibrium,  that  is,  bal 
ancing  each  other ;  thus,  iron,  steel,  nickel,  and  cobalt,  are 
such  bodies. 

MAGNETIZED  BODIES  also  contain  the  two  fluids,  but  the 
difference  between  them  and  magnetic  bodies  is,  that  in  the 
former  the  two  fluids  are  separated,  each  producing  an 
opposite  effect,  whilst  in  the  latter  the  fluids  are  combined 
and  produce  no  effect.  In  a  word,  magnetic  bodies  are 

What  is  the  boreal  fluid  t  The  austral  fluid  f  Which  turns  towards  the  north  f 
Why?  (348.)  What  is  the  first  law  of  magnetic  attraction  and  repulsion?  The 
second  law?  (  349.)  "What  is  a  Magnetic  Body  ?  Examples.  What  are  Magnetized 
Bodies? 


364  POPULAR     PHYSICS. 

capable  of  being  magnetized,  but  are  not  yet  magnets  ;  they 
present  neither  poles  nor  neutral  line. 

When  a  magnetic  substance  is  brought  into  contact  with 
one  of  the  poles  of  the  magnet,  as  the  boreal  pole,  for  ex 
ample,  the  latter,  acting  by  its  attraction  upon  the  austral 
fluid,  and  by  its  repulsion  upon  the  boreal  fluid,  separates 
them,  giving  rise  to  poles,  producing  a  real  magnet. 

If  a  magnetized 
bar  be  presented  to 
a  magnetic  body, 
as  an  iron  ring,  it 
converts  it  into  a 
magnet  in  the  man 
ner  just  described. 
If  a  second  ring  be 
presented  to  the 
first,  it  is  in  like 
manner  converted 
into  a  magnet,  and  so  on  for  a  third,  fourth,  &c.  The 
magnets  thus  formed  adhere  to  each  other,  as  shown  in 
Fig.  240.  If  the  bar  be  removed,  the  rings  cease  to  be 
magnets,  the  chain  falls  to  pieces,  and  the  rings  separate. 
^This  mode  of  exciting  magnetic  phenomena  is  called  mag 
netizing  by  induction.  According  to  the  theory  of  two 
fluids,  it  is  in  consequence  of  this  action  that  a  magnet  is 
capable  of  attracting  magnetic  bodies.  It  first  acts  by 
induction  to  convert  them  into  magnets,  and  then  it  attracts 
them  according  to  the  laws  laid  down  in  the  last  article. 

Fig.  241  represents  a  common  child's  toy.  A  small  swan  made 
of  glass  has  a  piece  of  iron  in  its  head,  and  on  presenting  to  it  a 
magnet,  the  swan  approaches  it,  swimming  along  the  surface  of  the 
water  upon  which  it  is  placed.  The  magnet  may  be  concealed  in  a 

How  are  magnets  produced  ?  Illustrate.  What  is  magnetic  induction  ?  Explain 
it  on  the  two  fluid  theory.  Explain  the  magnetic  swan. 


GENERAL     PROPERTIES     OP     MAGNETS. 


365 


Fig.  241 

piece  of  bread,  in  which  case  the  swan  seems  desirous  of  feeding 
upon  the  bread. 

The  Coercive  Force. 

35^0  The  force  required  to  separate  the  two  fluids  in  a 
magnetic  body  is  called  the  COERCIVE  FORCE. 

The  fluids  are  not  separable  with  equal  ease  in  all  bodies. 
In  some,  as,  for  example,  in  soft  iron,  they  yield  easily  and 
separate  at  once  ;  in  others,  as  in  hardened  steel,  for  exam 
ple,  the  fluids  yield  with  difficulty,  and  a  powerful  magnet 
is  required  to  effect  the  separation,  and  it  is  effected  only 
after  a  greater  or  shorter  length  of  time.  The  harder  and 
better  tempered  the  steel,  the  more  difficult  it  becomes  to 
separate  the  two  fluids. 

(  350.)  What  is  the  Coercive  Force  ?     IIo\v  i*  it  in  different  bodies  ? 


366 


POPULAR    PHYSICS. 


In  soft  iron  the  coercive  force  required  to  separate  the  fluids  is 
very  small,  in  hardened  steel  it  is  very  great.  Soft  iron  brought  in 
contact  with  a  bar  magnet  becomes  a  magnet  instantly,  and  on 
being  removed  returns  to  its  neutral  condition,  ceasing  to  be  a  mag 
net.  With  hardened  steel  the  reverse  is  the  case  ;  it  takes  con 
siderable  force  and  some  time  to  render  it  a  magnet,  and  on  being 
removed  from  the  bar  it  continues  to  be  a  magnet.  The  force  which 
resisted  the  separation  of  the  fluids  in  the  first  instance,  now  acts  to 
prevent  their  reunion,  so  that  the  steel  magnet  retains  its  magnetism 
for  a  long  time. 


II. TERRESTRIAL      MAGNETISM. COMPASSES. 


--S 


Directive   Force   of  Magnets. 

351.  When  a  permanent  magnet  is  balanced  so  that  it 
can  turn  freely  in  a  horizontal  direction,  it  assumes,  after  a 
few  oscillations,  a  determinate  direction,  which  is  very  nearly 
north  and  south. 

Fig.  242  shows  the  man 
ner  of  balancing  a  needle, 
and  indicates  the  north  and 
south  direction  which  it  as 
sumes.  In  this  figure,  as  in 
all  others  illustrating  the 
subject  of  magnetism,  the 
pole  which  contains  the  aus 
tral  fluid  is  designated  by 
the  letter  A,  whilst  that 
which  contains  the  boreal 
fluid  is  designated  by  the 


letter  B.  Fig.  242. 

It  will  be  noticed  that  it 
is  the  austral  pole  which  turns  towards  the  north,  and  the 


Illustrate.    (351.)  What  direction  does  a  freo  magnet  fake?    How  is  a  needle 
balanced  ?    In  what  other  way  may  it  be  balanced  ? 


TERRESTRIAL     MAGNETISM.  367 

boreal  pole  which  turns  towards  the  south,  the  reason  of 
which  will  be  seen  hereafter. 

If,  instead  of  mounting  the  needle  on  a  pivot,  it  be 
attached  to  a  piece  of  cork  and  placed  in  a  vessel  of  water, 
so  that  the  needle  may  float  in  a  horizontal  position,  it  will 
turn  itself  slowly  around  and  come  to  rest  in  the  same 
general  direction  as  though  it  were  balanced  on  a  pivot. 
In  this  experiment  it  will  be  found  that  the  needle  once  in 
the  meridian,  does  not  advance  either  towards  the  north  or 
south.  Hence  we  infer  that  the  force  exerted  upon  the 
needle  is  simply  a  directive  one. 

The  force  which  causes  a  movable  magnet  to  direct  itself 
north  and  south  is  called  the  directive  force. 

Since  the  phenomenon  described  takes  place  at  all  points  of  the 
earth's  surface,  the  earth  has  been  regarded  as  an  immense  magnet, 
having  its  boreal  and  austral  poles  near  the  north  and  south  poles 
of  the  earth,  and  a  neutral  line  near  the  equator.  This  immense 
magnet  acting  upon  the  smaller  magnets  described,  would  produce 
all  of  the  effects  observed.  When  we  come  to  explain  the  action  of 
electric  currents,  it  will  be  seen  that  there  is  another  explanation  of 
the  directive  power  of  the  earth. 

Magnetic   Meridian.  —  Declination.  —  Variations. 

352.  When  a  balanced  magnetic  needle  comes  to  a  state 
of  rest,  it  points  out  the  line  of  magnetic  north  and  south. 
If  a  plane  be  passed  through  the  needle  in  this  position  and 
the  centre  of  the  earth,  it  is  called  the  plane  of  the  mag 
netic  meridian,  or  simply  the  magnetic  meridian. 

This  does  not,  in  general,  coincide  with  the  plane  of  the 
true  meridian,  which  is  determined  by  a  plane  passing 
through  the  place  and  the  axis  of  the  earth.  The  angle 
which  the  magnetic  meridian  at  any  place  makes  with  the 

How  is  it  shown  that  the  magnetic  force  is  simply  directive  ?  What  is  the  directive 
force?  Why  has  the  earth  been  regarded  as  a  magnet?  Where  are  its  poles  f 
(  352  )  What  is  the  magnetic  meridian  ?  What  is  the  declination  of  the  needle  ? 


368  POPULAR     PHYSICS. 

true  meridian  of  the  same  place  is  called  the  declination  of 
the  needle.  In  short,  the  declination  of  the  needle  is  its 
variation  from  true  north  and  south.  This  is  different  at 
different  places  on  the  earth,  and  even  at  the  same  place  at 
different  times. 

When  the  north  end  of  the  needle  points  to  the  east  of 
true  north,  the  declination  is  said  to  be  to  the  east  /  when  to 
the  west  of  true  north  the  declination  is  said  to  be  to  the 
west. 

There  is  a  line  running  from  near  Cleveland,  Ohio,  to 
Charleston,  S.  C.,  along  which  the  needle  points  to  the  true 
north ;  this  is  called  a  line  of  no  declination. 

The  line  of  no  declination  is  travelling  slowly  to  the  westward  at 
a  rate  which  would  carry  it  around  the  globe  in  about  1000  years. 
For  all  points  of  the  United  States  east  of  the  line  of  no  declination, 
the  declination  of  the  needle  is  to  the  west ;  for  all  points  to  the 
west  of  it,  the  declination  is  to  the  east  ;  that  is,  the  north  end  of 
the  needle  in  all  cases  is  inclined  towards  the  line  of  no  declination. 

For  all  points  in  the  United  States  to  the  east  of  the  line  of  no 
declination,  the  declination  is  slowly  increasing,  whilst  for  all  points 
to  the  west  of  it,  the  declination  is  slowly  decreasing. 

Besides  this  slow  change  in  declination,  the  needle  under 
goes  slight  changes,  some  of  which  are  pretty  regular  and 
others  very  irregular.  In  our  latitude  the  north  end  of  the 
needle  moves  towards  the  west  during  the  early  part  of 
every  day,  through  an  angle  of  10  or  15  minutes,  and  moves 
back  again  during  the  latter  part  of  the  day.  This  is  called 
the  diurnal  variation.  In  the  southern  hemisphere  this 
motion  is  reversed.  There  is  also  a  small  change  of  similar 
character  which  takes  place  every  year,  called  the  annual 
variation. 


When  is  it  to  the  east?  To  the  west?  What  is  the  line  of  no  declination  ?  HotA 
does  this  line  move ?  At  what  rate  ?  Where  is  the  declination  to  the  west  ?  To  the 
east?  How  does  the  declination  vary  in  the  United  States?  What  is  the  diurnal 
variation  ?  The  annual  variation  ? 


COMPASSES.  369 

Irregular  changes  are  called  perturbations.  They  usually  take 
place  during  thunder  storms,  during  the  appearance  of  the  aurora 
borealis.  and  in  general,  when  there  is  any  sudden  change  in  the 
electrical  condition  of  the  atmosphere. 

The  Compass. 

353.  The  property  possessed  by  magnets  of  arranging 
themselves  in  the  magnetic  meridian  has  been  utilized  in  the 
construction  of  COMPASSES. 


Fig.    243. 

Fig.  243  represents  a  compass.  It  consists  of  a  compass- 
box,  having  a  pivot  at  its  centre,  on  which  is  poised  a  delicate 
magnetic  needle.  Around  the  rim  of  the  box  is  a  graduated 
circle,  whose  diameter  is  somewhat  less  than  the  length  of 
the  needle,  and  of  which  the  pin  is  the  centre.  The  pin  is 
of  hard  steel,  carefully  pointed  ;  a  piece  of  hard  stone  is  let 


What  are  perturbations?     Illustrate.    (  353.)  What  is  a  Compass?    Describe  it. 

16* 


370  POPULAR     PHYSICS. 

into  the  needle,  in  which  is  a  conical  hole  to  rest  upon  the 
pivot,  to  diminish  the  friction  between  the  needle  and  its 
support.  In  addition  to  the  graduation  on  the  circle,  the 
bottom  of  the  box  is  divided  into  sixteen  equal  parts,  in 
dicating  the  points  of  the  compass. 

This  instrument  under  various  forms  is  used  for  a  great  variety  of 
purposes.  It  is  used  in  navigation,  in  surveying,  and  is  of  im 
portance  to  the  traveller  and  explorer,  to  say  nothing  of  its  use  in 
mining. 

The  magnetic  declination  at  any  place  may  easily  be  found  when 
the  true  meridian  is  known.  Let  the  compass  be  so  placed  that  the 
line,  NS,  coincides  with  the  true  meridian,  then  when  the  needle 
comes  to  rest,  the  reading  under  the  head  of  the  needle  will  be  the 
declination  required.  In  the  figure,  if  we  suppose  NS  to  be  in  the 
true  meridian,  the  declination  is  19°  west. 

The  Dipping  Needle. 

354.  When  a  steel  needle,  mounted  as  shown  in  Fig. 
242,  is  carefully  balanced  before  being  magnetized,  it  is 
found,  after  being  magnetized,  to  incline  downwards  or  to 
dip.  This  dip  is  towards  the  north  in  our  latitude,  that  is, 
the  north  end  of  the  needle  dips  or  inclines.  The  defect  of 
dipping  in  the  compass  is  remedied  by  making  the  other  end 
of  the  needle  a  little  heavier,  by  adding  a  movable  weight, 
as  a  piece  of  wire  wround  round  the  needle,  and  capable  of 
sliding  along  it. 

To  show  the  dip  and  to  measure  it,  the  needle  is 
mounted  in  the  way  indicated  in  Fig.  244.  The  needle  is 
suspended  on  a  horizontal  axis,  so  that  it  can  move  up  and 
down  freely,  and  the  amount  of  the  dip  is  indicated  by  a 
graduated  circle  or  quadrant.  The  dip  indicated  in  the 
figure  is  54°,  which  is  the  angle  made  by  the  needle  with 


What  is  it*  use  ?  Hoic  is  the  magnetic  declination  found  at  any  place  ? 
(354)  What  is  a  dipping  needle?  How  is  the  compass  needle  prevented  from 
dipping  ?  How  is  the  dip  shown  and  measured  ? 


METHODS     OF     MAGNETIZING     BODIES. 


371 


the  horizon.  At  any  place  the 
dip  will  be  the  greatest  possi 
ble  when  the  needle  vibrates 
in  the  plane  of  the  magnetic 
meridian. 

The  dip  varies  in  passing  from 
place  to  place,  increasing  as  we  ap 
proach  the  magnetic  poles  of  the 
earth,  where  the  dip  is  90° ;  that  is, 
the  needle  is  perpendicular  to  the 
horizon. 

The  dip  is  subject  to  irregularities 
corresponding  to  those  of  the  declina 
tion.  The  amount  of  the  dip  is  an 
important  element  in  forming  a  cor 
rect  notion  of  the  laws  of  terrestrial 
magnetism,  and  for  this  reason  many 
observations  have  been  made  and 
are  still  making,  to  determine  it  at 
different  places,  and  at  different 
times  at  the  same  place! 


Fig.  244. 


III.  —  METHODS   OP   IMPARTING   MAGNETISM. 

Magnetizing  by  Terrestrial  Induction. 

355.  To  MAGNETIZE  a  body  is  to  impart  to  it  the 
properties  of  a  magnet ;  that  is,  to  impart  to  it  the  proper 
ty  of  attracting  magnetic  bodies. 

The  only  substances  that  can  be  permanently  magnetized, 
are  steel  and  the  compound  oxide  of  iron,  which  constitutes 
the  loadstone.  A  body  capable  of  being  magnetized  may 
be  converted  into  a  magnet  by  the  inductive  influence  of 


IIow  docs  the  dip  vary  f    Is  it  subject,  to  irregularities  f    (355.)  What  la  meant 
by  magnetizing  a  body  ?    What  substances  can  be  permanently 


372  POPULAR    PHYSICS. 

the  earth,  or  more  rapidly  by  being  rubbed  by  another 
magnet,  or  finally,  by  the  action  of  electricity,  in  which  case 
the  operation  is  instantaneous. 

The  magnetic  ores  of  iron  may  exist  as  magnets  in  the  natural 
state,  or  they  may  possess  no  trace  of  magnetic  action.  But  they 
are  highly  susceptible  to  magnetic  influence,  and  once  magnetized, 
they  retain  their  magnetic  action  by  virtue  of  their  strong  coercive 
force. 

Natural  magnets  owe  their  magnetism  to  the  slow  action 
of  the  earth,  which  separates  the  two  fluids  in  them.  The 
magnetic  action  of  the  earth  is  so  great  as  to  be  used  suc 
cessfully  in  forming  artificial  magnets. 

To  use  this  principle,  we  place  a  thin  bar  of  iron  in  the 
magnetic  meridian  and  incline  it  to  the  horizon  by  an  angle 
equal  to  the  dip.  In  this  position  the  earth  acts  upon  it  by 
induction,  driving  the  austral  fluid  to  the  lower  end  (in  our 
latitude),  and  the  boreal  fluid  to  the  upper  end. 

The  magnetism  thus  induced  is  only  temporary,  for  if  the 
bar  be  moved  from  its  position,  the  two  fluids  return  to  a 
state  of  equilibrium.  If,  however,  when  the  bar  is  in  posi 
tion,  it  be  struck  smartly  by  a  hammer,  or  if  it  be  violently 
twisted,  sufficient  coercive  force  may  be  developed  to  retain 
the  induced  magnetism  for  a  time. 


Magnetizing  by  Friction. 

356.  Bars  of  steel,  and  needles  for  compasses,  are  usually 
magnetized  by  rubbing  them  with  other  magnets.  The 
three  methods  are  called  the  methods  by  single  touch,  by 
separate  touch,  and  by  double  touch. 

To  magnetize  a  steel  bar  by  single  touch,  we  hold  the 


Are  the  magnetic  ores  of  iron  always  magneto  f  To  what  is  the  natural  mag 
netization  of  these  ores  due  ?  How  are  bars  magnetized  by  this  principle?  (356-) 
How  may  bars  of  steel  be  magnetized  ?  Explain  the  method  of  single  touch. 


METHODS     OF    MAGNETIZING    BODIES.  373 

body  to  be  magnetized  in  one  hand,  and  with  the  other  we 
pass  over  it  a  powerful  bar  magnet,  as  shown  in  Fig.  245. 
After  several  repetitions  of  this  process,  the  steel  is  found  to 
possess  all  the  properties  of  a  magnet.  These  properties 


Fig.  245. 

are  the  more  durable  in  proportion  to  the  hardness  of  the 
steel. 

To  magnetize  a  steel  bar  by  separate  touch,  we  rub  J.t  in 
one  direction  with  one  pole  of  a  magnetized  bar,  and  in  tae 
opposite  direction  with  the  opposite  pole. 

To  magnetize  a  body  by  double  touch,  we  make  use  of 
two  magnetized  bars,  which  are  placed  with  their  opposite 
poles  in  contact  with  the  bar  at  its  middle  point,  being  orly 
separated  by  a  small  interval,  as  shown  in  Fig.  246  ;  the 
combined  bars  are  then  moved  alternately  in  opposite  direc 
tions  to  the  two  ends  of  the  bar,  and  the  operation  is 
repeated  several  times.  Care  must  be  taken  to  apply  the 
same  number  of  touches  to  each  end  of  the  bar. 

Of  separate  touch.    Of  double  touch. 


POPULAR     PHYSICS. 


Fig.  246. 

The  method  of  magnetizing  by  electricity  will  be  treated 
of  under  the  head  of  electrical  currents. 


Bundles   of  Magnets. — Armatures. 

A  BUNDLE  OF  MAGNETS  consists  of  a  group  of 
magnetized  bars  united,  so  that  their  poles  of  the  same 
name  may  be  coincident. 

Sometimes  these  bundles  are  composed  of  straight  bars, 
like  that  shown  in  Fig.  245,  and  sometimes  they  are  curved 
in  the  shape  of  a  horse-shoe,  as  shown  in  Fig.  247. 

Magnets,  if  abandoned  to  themselves,  would  lose  in  a 
short  time  much  of  their  power ;  hence  it  is,  that  arma 
tures  are  employed. 

An  ARMATURE  is  a  piece  of  soft  iron,  placed  in  contact 
with  the  poles  of  a  magnet.  Thus,  ab,  in  Fig.  247,  is  an 
armature. 

(351.}  What  is  a  Bundle  of  Magnets?    What  is  an  Armature  ? 


METHODS     OF     MAGNETIZING     BODIES. 


375 


The  poles,  acting  by  induction  upon  the  armature,  convert 
a  into  an  austral,  and  b  into  a 
boreal  pole.  These  two  poles  re 
acting  upon  the  poles  of  the  mag 
net,  AJ3,  prevent  the  recomposi- 
tion  of  the  two  fluids,  and  thus 
preserve  its  magnetism.  The  ar 
mature  is  sometimes  called  a 
keeper. 


If  weights  be  attached  to  the  keeper 
till  it  separates  from  the  magnet,  we 
can,  from  the  number  of  pounds  ap 
plied,  judge  of  the  power  of  the  mag 
net. 

For  many  kinds  of  magnetic  experi 
ment  the  horse-shoe  form  is  preferable. 
It  is  also  the  form  best  adapted  to  the 
application  of  an  armature  or  keeper. 

The  most  powerful  horse-shoe  mag 
nets  are  formed  by  means  of  electrical 
currents.  Magnets  of  this  kind  have 
been  constructed  by  Prof.  HENRY,  of  the 
Smithsonian  Institution,  capable  of  sus 
taining  a  weight  of  more  than  a  ton 
and  a  quarter. 


Fig.  247. 


A  keeper?    How  can  we  judge  of  the  power  of  a  magnet?     What  are  the  ad* 
vantages  of  the  horse-shoe  magnet  T 


CHAPTER   YIII. 

STATICAL          ELECTRICITY. 

I. — FUNDAMENTAL       PRINCIPLES. 

Definition  of  Electricity. 

358.  ELECTRICITY,  as  a  science,  is  that  branch  of  Physics 
which  treats  of  the   laws  of  attraction  and  repulsion  ex 
hibited  by  bodies  under  certain  circumstances.     Such  phe 
nomena  are  called  electrical  phenomena.     The  name  elec 
tricity  is  derived  from  the  Greek  elektron,  which  means 
amber. 

Discovery  of  Electrical  Properties. 

359.  Six   hundred    years    before    the    commencement    of    the 
Christian  era,  THALES.  of  Miletus,  knew  that  when   yellow  amber 
was  vigorously  rubbed  with  wool,  it  acquired  the  property  of  attract 
ing   light    bodies,  such   as   small   pieces  of  paper,   barbs  of  quills, 
straws  and  the  like.     Comparing  this  action   to  suction,  Ihe  ancients 
said   that  amber  had  a   power  of  suction,  and  sucked  light  bodies 
lowards  it.     In  consequence  of  the  rarity  of  amber,  whose  origin  is 
even  in  our  day  unknown,  they  went  so  far    as  to  say,  that  it  was 
formed  from   the  tears  of  an  Indian  bird,  grieved  at  the  death  of 
King  MELEAGER. 

Six  centuries  later.  PLINY,  an  eminent  Roman  naturalist,  writes  : 
'•  When  the  friction  of  the  fingers  imparts  heat  and  life  to  yellow 
amber,  it  attracts  straws,  just  as  the  magnet  attracts  iron/'  This 


(  358.)  Define  Electricity  as  a  science.    What  are  electrical  phenomena  ?    Whence 
the  name  ?    (  3  59.)  Give  an  outline  of  the  history  of  electrical  discoveries. 


FUNDAMENTAL    PRINCIPLES     OF    ELECTRICITY. 


377 


was  all  of  the  knowledge  had  on  the  subject  until  the  end  of  the 
sixteenth  century,  when  WILLIAM  GILBERT,  an  Englishman,  called 
anew  the  attention  of  scientific  men  to  the  properties  of  amber,  and 
showed  that  a  great  number  of  other  substances,  such  as  glass,  resin, 
silk,  sulphur,  and  the  like,  acquired  the  power  of  attracting  light 
bodies,  on  being  rubbed  with  woolen  cloth  or  cat's  skin. 

To  repeat  these  experiments,  rub  a  tube  of  glass  or  a  stick  of 
sealing-wax  with  a  piece  of  woolen  cloth,  then  present  them  to  light 
bodies,  as  shreds  of  gold  leaf,  barbs  of  quills,  or  fragments  of  paper, 
and  the  latter  will  be  seen  to  approach  and  adhere  to  the  excited 
glass  or  sealing-wax.  The  manner  of  making  these  experiments  is 
indicated  in  Fig.  248. 

It  will  be  seen  here 
after,  that  resin  and 
other  substances  named 
above,  not  only  develop 
forces  of  attraction 
when  rubbed,  but  also 
they  become  luminous, 
emit  sparks,  and  dis 
play  a  number  of  other 
properties,  all  of  which 
are  known  as  electrical 
phenomena. 

Since  the   beginning 

of  the  seventeenth  century  the  progress  of  discovery  in  electricity  has 
been  rapid,  and  a  multitude  of  new  facts  have  been  developed,  which 
have  been  so  well  studied  as  to  form  a  very  extensive  branch  of 
natural  science. 

Sources  of  Electricity. 

36O.  The  sources  of  electricity  may  be  divided  into 
three  classes :  Mechanical,  Physical,  and  Chemical. 

The  mechanical  sources  are:  friction,  pressure,  and  separ 
ation  of  the  molecules  of  bodies.  When  a  piece  of  sugar  is 
broken  suddenly  in  a  dark  room,  a  feeble  light  is  observable, 


Fig.  248. 


Explain  GILBERT'S  experiments,  and  the  manner  of  making  them.    ( 360.)  What 
are  the  principal  sources  of  electricity?    What  are  the  mechanical  sources  ? 


378  POPULAR    PHYSICS. 

which  is  due  to  the  development  of  electricity  at  the  mo 
ment  of  separating  the  molecules. 

The  physical  sources  are  variations  of  temperature,  and 
the  like.  Some  minerals,  particularly  tourmaline  and  topaz, 
manifest  electrical  phenomena  on  being  heated  or  cooled. 

The  chemical  sources  are  chemical  compositions  and  de 
compositions  of  bodies.  Metals,  like  zinc,  iron,  and  coppei; 
when  plunged  into  acids,  are  attacked  by  them,  forming 
compounds  known  as  salts.  During  these  combinations 
considerable  quantities  of  electricity  are  developed. 

The  most  powerful  of  the  causes  of  electricity  are  friction  and 
chemical  action.  These  will  be  studied  in  their  order. 

Electroscope.— Electrical  Pendulum. 

361.  An   ELECTROSCOPE  is   an  apparatus   for  showing 
when  a  body  is  electrified. 

The  most  simple  electroscope  is  the  ELECTRICAL  PEN 
DULUM,  which  consists  of  a  small  ball  of  elder  pith,  suspended 
by  a  fine  silk  thread,  as  shown  in  Fig.  249.  The  thread  is 
fastened  to  the  upper  end  of  a  stem  of  copper,  which  stem 
has  a  support  of  glass. 

To  ascertain  whether  a  body  is  electrified  or  not,  the 
pendulum  is  presented  to  it ;  if  it  is  electrified,  the  pith  ball 
will  be  attracted,  otherwise  not.  When  the  quantity  of 
electricity  is  too  small  to  produce  sensible  attraction  upon 
the  pith  ball,  more  delicate  instruments  are  sometimes  em 
ployed,  called  electrometers. 

Two  kinds   of  Electricity. 

362.  That  there  are  two  kinds  of  electricity,  may  be 
shown  by  the  action  of  glass  and  resinous  bodies,  after  being 
rubbed,  upon  pith  balls. 

What  is  the  chief  physical  source?  The  chemical  sources?  What  is  the  most 
powerful  cause  of  electricity  ?  (361.)  What  is  an  Electroscope?  Describe  the 
Electrical  Pendulum.  How  used  ?  (  362-)  How  many  kinds  of  electricity  are  there  ? 


FUNDAMENTAL    PRINCIPLES     OF     ELECTRICITY. 


379 


If  a  tube  of  glass  be  rubbed  with  a  piece  of  cloth,  and 
then  presented  to  the  electrical  pendulum,  the  pith  ball  will 
fit  first  be  attracted,  and  after  a  short  time  it  will  be  repelled, 
as  shown  in  Fig.  250.  The  ball  is  then  charged  with  the 
same  kind  of  electricity  as  that  in  the  glass. 


Fig.  249. 


Fig.  250. 


If  now  a  piece  of  a  resinous  body,  as  sealing-wax,  be 
rubbed  with  cloth  and  brought  near  the  excited  pith  ball, 
the  latter  is  immediately  attracted  to  the  former.  In  like 
manner,  if  the  sealing-wax  be  first  presented  to  the  pen 
dulum,  it  will  be  attracted  and  then  repelled.  If  then  the 
glass  be  brought  near  the  pith  ball,  attraction  will  be  ob 
served.  This  shows  that  the  action  of  electricity,  as  devel 
oped  in  glass  and  resin,  is  different,  the  one  repelling  when 
the  other  attracts.  This  fact  was  discovered  by  DUFAY, 
in  1734. 

The  electricity  developed  in  rubbing  glass  with  a  piece 

How  is  it  shown  that  there  are  two  kinds  ? 


380  POPULAR     PHYSICS. 

of  silk,  has  been  named  vitreous  electricity,  that  developed 
by  rubbing  resin  or  sealing-wax  with  the  silk,  has  beev 
named  resinous  electricity. 

Hypothesis   of  two  Electrical  Fluids. 

363.  The  discovery  of  DUFAT  gave  rise  to  the  theory 
of  two  electrical  fluids,  which  in  unexcited  bodies  exist  in  a 
state  of  combination,  forming  what  is  called  a  neutral  fluid. 
The  earth  is  regarded  as  a  great  reservoir  of  this  fluid,  which 
has  of  itself  no  obvious  properties  ;  hence  bodies  which 
only  contain  it  are  said  to  be  neutral.  If  by  friction,  chem 
ical  action,  or  other  cause,  the  neutral  fluid  is  decomposed, 
and  the  two  fluids  separated,  electrical  phenomena  are  at 
once  developed. 

These  two  fluids  were  at  first  named  the  vitreous,  and  the 
resinous  fluids,  but  more  recently  they  have  been  called 
the  positive,  and  the  negative  fluids ;  the  vitreous  being 
called  positive,  and  the  resinous  negative.  These  names 
were  adopted  by  FRANKLIN  the  better  to  express  their  op 
posite  characters.  The  positive  fluid  is  often  indicated  by 
this  sign,  -f  •>  (plus),  and  the  negative  fluid  by  this  sign,  —  , 
(minus.) 

The  hypothesis  of  two  fluids  was  first  made  by  SYMNER,  and  ac 
cording  to  it  the  development  of  electricity  consists  in  separating  the 
two  fluids.  When  glass  is  rubbed  with  silk,  the  positive  fluid  of  the 
two  goes  to  the  glass,  whilst  the  negative  fluid  goes  to  the  silk. 
When  sealing-wax  is  rubbed  with  silk,  the  reverse  is  the  case,  the 
negative  fluid  goes  to  the  resinous  body  and- the  positive  fluid  to  the 
silk. 

It  is  to  be  observed  that  all  of  the  phenomena  can  be  equally  well 
explained  by  the  theory  of  a  single  fluid.  This  is  the  theory  of 
FRANKLIN,  and  if  we  adhere  to  the  hypothesis  of  two  fluids,  it  is 
simply  because  it  is  more  easily  applied  than  that  of  one  fluid. 

What  are  they  called  ?  (  363.)  What  is  the  neutral  fluid  ?  When  are  electrical 
phenomena  produced  ?  What  other  names  are  given  to  the  two  fluids  ?  How  are 
they  indicated  ?  Explain  in  detail  the  two  fluid  hypothesis.  Who  is  the  author 
of  the  one  fluid  hypothesis? 


FUNDAMENTAL     PRINCIPLES     OF     ELECTRICITY.  381 


Laws   of  Electrical  Attraction  and  Repulsion. 

364.  The   following    laws    have    been    deduced  from 
theory,  and  confirmed  by  experiment : 

1.  Fluids  of  the  same  name  repel  each  other  ;  fluids  of 
opposite  names  attract  each  other. 

2.  The  intensities  of  the  attractions  and  repulsions  vary 
inversely  as  the  square  of  the  distances  between  them. 

Conductors.  —  Insulators. 

365.  CONDUCTORS,  or  conducting  substances,  are  those 
which  permit  electricity  to  pass  through  them. 

INSULATORS,  or  non-conducting  substances,  are  those 
which  do  not  permit  electricity  to  pass  through  them. 

GRAY  observed  that  electrified  bodies  returned  instantly  to  a 
neutral  state  when  brought  into  contact  with  the  earth,  or  when 
placed  upon  supports  of  metal,  wood,  stone,  or  any  moist  substance 
whatever.  He  also  observed  that  they  remained  in  an  electrified 
condition  for  a  long  time  when  placed  upon  supports  of  glass,  resin, 
sulphur,  or  when  suspended  by  silken  cords.  From  these  facts,  he 
concluded  that  metals,  wood,  stone,  and  the  like,  permitted  the 
electricity  to  pass  freely  through  them,  whilst  glass,  resin,  sulphur, 
and  the  like,  opposed  its  passage.  He  also  inferred  that  the  latter 
class  of  bodies  was  not  entirely  incapable  of  conducting  electricity, 
but  that  they  were  extremely  poor  conductors.  When  an  electrified 
body  is  surrounded  by  non-conductors  it  is  said  to  be  insulated,  and 
any  non-conducting  support  of  an  electrified  body  is  therefore  called 
an  insulator. 

The  best  conductors  of  electricity  are  the  metals;  after 
these  come  plumbago,  well  calcined  carbon,  acid  and  saline 

(  3G4.)  What  is  the  first  law  of  attraction  and  repulsion?  The  second  law? 
(C65.)  What  are  Conductors ?  Insulators  or  non-conductors?  What  observations 
were  made  l>y  GRAY  ?  When  is  a  body  insulated  t  What  are  the  best  conductors  ? 
Next  in  order  ? 


382  POPULAR     PHYSICS. 

solutions,  water  either  in  a  liquid  or  vaporous  form,  the  hu 
man  body  or  animal  tissues,  vegetable  substances,  and  in 
general,  all  moist  or  humid  substances. 

The  worst  conductors,  or  best  non-conductors-  are  resins, 
gums,  india-rubber,  silk,  glass,  precious  stones,  spirits  of 
turpentine,  oils,  air,  and  gases  when  perfectly  dry. 

Methods  of  Electrifying  Bodies. 

366.  Non-conducting  bodies  are  electrified  only  by 
friction,  but  conductors  may  be  electrified  either  by  friction, 
by  contact,  or  by  induction. 

In  order  to  electrify  a  metal  it  must  be  insulated ;  that  is, 
it  must  be  surrounded  by  non-conducting  bodies,  and  it 
must  be  rubbed  by  an  insulated  body. 

This  may  be  effected  by  mounting  the  metal  upon  a  stand  of  glass 
and  rubbing  it  with  a  non-conductor,  such  as  a  piece  of  silk.  Were 
the  metal  not  insulated,  the  electricity  would  flow  off  to  the  earth  as 
fast  as  generated,  and  were  the  rubbing  body  not  a  non-conductor, 
the  electricity  would  flow  off  through  the  hands  and  arms  of  the 
experimenter. 

The  method  of  electrifying  by  contact  depends  upon  the 
property  of  conductibility.  If  a  conductor  is  brought  in 
contact  with  an  electrified  body,  a  portion  of  the  electricity 
of  the  latter  at  once  flows  into  the  former  body.  If  the  two 
bodies  are  exactly  alike,  the  electricity  will  be  equally  dis 
tributed  over  both.  If  they  differ  in  size  or  in  shape,  the 
electricity  will  not  be  equally  distributed  over  both. 

The  method  of  electrifying  bodies  by  induction  is  similar 
to  that  of  magnetizing  bodies  by  induction,  and  will  be 
treated  of  hereafter. 


The  worst  conductors?    (366.)  How  are  non-condnctors  electrified?     Can  con 
ductors  be  electrified  by  friction?    ffowf    How  are  bodies  electrified  by  contact! 


FUNDAMENTAL    PRINCIPLES     OF     ELECTRICITY. 


383 


Accumulation  of  Electricity  on  the  Surface  of  Bodies. 

367.  Experiment  shows  that  when  a  body  is  electrified, 
the  electricity  all  goes  to  the  surface  of  the  body,  where  it 
exists  in  a  thin  layer,  tending  continually  to  escape.  It 
actually  does  escape  as  soon  as  it  finds  an  outlet  through  a 
conducting  body. 


Fig.  251. 

Of  the  various  experiments  intended  to  show  this  fact,  we 
salect  one  that  was  first  performed  by  COULOMB.  He 
mounted  a  copper  sphere  upon  an  insulating  rod  of  glass,  as 
shown  in  Fig.  251.  He  then  provided  two  hollow  hemis 
pheres  also  of  copper,  which,  when  put  together,  exactly 


(  ?67.)  Where  is  the  electricity  of  a  body  found  ?    Explain  COULOMB'S  experiment. 


384:  POPULAR     PHYSICS. 

fitted  the  first  sphere,  and  these  he  insulated  by  attaching 
them  to  glass  handles.  Having  placed  the  hemispheres  so 
as  to  cover  the  solid  sphere,  he  brought  the  whole  apparatus 
in  contact  with  an  electrified  body  till  it  was  fully  charged. 

On  removing  the  apparatus  from  the  electrified  body,  he 
separated  the  two  hemispheres  abruptly,  and  applied  to  eacli 
in  turn  the  electrical  pendulum,  when  he  found  that  both 
were  electrified.  On  testing  the  solid  sphere  in  like  manner, 
he  could  discover  no  trace  of  electricity  ;  in  other  words,  it 
was  perfectly  neutral. 

In  taking  away  from  the  body  its  outer  coating,  he  had 
removed  every  particle  of  its  electricity,  which  proved  that 
the  electricity  was  entirely  upon  the  surface. 

Another  fact  which  indicates  the  same  conclusion  is,  that  a  hollow 
and  a  solid  sphere  of  the  same  size  and  of  the  same  material,  will  be 
charged  with  exactly  the  same  quantity  of  electricity  when  made  to 
communicate  with  the  same  electrical  source. 

When  the  electric  fluid  is  accumulated  upon  the  surface 
of  a  body,  it  tends  to  escape  with  a  certain  force,  which  is 
named  the  tension. 

The  tension  augments  with  the  quantity  of  electricity  accumu 
lated.  So  long  as  it  does  not  pass  a  certain  limit,  it  is  held  by  the 
resistance  of  the  air,  but  if  the  tension  passes  this  limit,  the  elec 
tricity  escapes  with  a  crackling  noise  and  a  brilliant  light  called  the 
electric  spark.  In  moist  air  the  tension  is  always  feeble,  because 
the  electricity  is  slowly  conveyed  away  by  the  moisture.  In  a 
vacuum,  there  is  no  resistance  to  the  escape  of  electricity,  and  the 
tension  is  nothing.  The  electricity  in  this  case  flows  off  as  fast  as 
generated,  with  a  feeble  light. 

Influence   of  the  Forms  of  Bodies.  —  Power  of  Points. 

368.  The  distribution  of  electricity  over  the  surfaces  of 
bodies  depends  upon  their  form.  If  a  body  is  spherical,  the 

What  fact  confirms  COULOMB'S  conclusion  ?  What  is  the  tension  ?  What  is  the 
•electric  spark  ?  Why  is  the  tension  feelle  in  moist  air  f  In  a  vacuum  f  (  368.) 
What  effect  has  the  form  of  a  body? 


FUNDAMENTAL     PRINCIPLES     OF     ELECTRICITY. 


385 


fluid  is  equally  distributed,  as  may  be  shown  by  an   instru 
ment  called  a  proof-plane. 

The  proof-plane  consists  of  a  disk  of  gilt  paper  attached  to  the  end 
of  a  rod  of  gumlac.  which  insulates  well.  Taking  the  rod  in  the 
hand  as  shown  in  Fig.  252,  it  is  applied  successively  at  different 
points  of  the  electrified  surface,  and  after  each  contact  it  is  presented 
to  the  electrical  pendulum. 


f\ 


If  the  electrified  body  is  a  sphere,  the  same  amount  of 
attraction  for  the  pith  ball  is  shown,  wherever  the  contact 
may  be  made ;  this  shows  that  the  proof-plane  is  equally 
charged  at  every  point  of  the  sphere,  and  consequently  it  is 
inferred  that  the  distribution  is  uniform  over  the  whole 
surface. 

When  the  body  is  elongated  and  pointed,  as  in  Fig.  252, 
different  results  are  obtained.  In  this  case  the  proof-plane 


What  is  a  proof-plane  f    How  used  ? 
17 


386  POPULAR    PHYSICS. 

is  more  highly  charged  at  the  sharp  end  of  the  body  than 
at  any  other  point,  showing  a  larger  amount  of  electricity  at 
the  point  than  elsewhere.  In  general,  it  may  be  shown  that 
the  greater  the  curvature  of  a  surface  at  any  part,  that  is, 
tha  nearer  it  approaches  a  point,  the  greater  will  be  the 
accumulation  of  electricity  there.  This  shows  that  elec 
tricity  tends  to  accumulate  at,  or  to  flow  towards  the 
pointed  portions  of  bodies. 

The  accumulation  of  electricity  at  points  gives  rise  to  a  high 
tension,  which  is  sufficient  to  overcome  the  resistance  of  the  air  and 
to  give  rise  to  an  escaping  current.  In  fact,  metallic  bodies  of  a 
pointed  shape  soon  lose  the  electricity  imparted  to  them,  and  often 
the  escaping  current  may  be  felt  by  placing  the  hand  in  front  of  the 
point.  If  the  flow  takes  place  in  a  darkened  room,  it  may  be  dis 
covered  by  a  feathery  jet  of  faint  light. 

The  property  of  points,  or  the  power  of  points,  as  it  is  called,  was 
noticed  by  FRANKLIN  and  made  use  of  by  him  in  his  theory  of 
lightning-rods. 

II. PRINCIPLE       OF      INDUCTION. ELECTRICAL       MACHINES. 

Induction. 

369.  If  an  insulated  conductor  in  a  neutral  state  is 
brought  near  an  electrified  body,  the  fluid  of  the  latter 
acting  upon  that  of  the  former,  decomposes  it,  repelling  the 
fluid  of  the  same  name,  and  attracting  that  of  a  contrary 
name.  This  operation  is  called  INDUCTION,  and  it  may  take 
place  not  only  at  considerable  distances,  but  also  through 
non-conducting  bodies,  such  as  air,  glass,  and  the  like. 

The  method  of  electrifying  bodies  by  induction  is  shown 
in  Fig.  253.  On  the  right  of  the  figure  is  the  prime  con 
ductor  of  an  electrical  machine,  which,  as  we  shall  see  here 
after,  is  charged  with  the  positive  fluid.  On  the  left  is  a 

What  effect  £as  a  pointed  form  ?  Discuss  th$  power  of  po,in\s.  (  369.)  What  ia 
Induction  \ 


INDUCTION. 


387 


metallic  cylinder  with  spherical  ends,  and  supported  by  a 
rod  of  glass.  Attached  to  its  lower  surface,  at  intervals,  are 
pairs  of  pith  ball  pendulums,  supported  by  threads  of  some 
conducting  substance. 


Fig.  268. 

When  the  cylinder  is  brought  slowly  towards  the  electri 
cal  machine,  we  see  the  pith  balls  repel  each  other  and 
diverge.  This  divergence  is  unequal  at  different  points, 
being  greatest  near  the  extremities  of  the  cylinder  ;  towards 
the  middle  of  the  cylinder  the  pith  balls  remain  in  contact 
without  repelling  each  other.  We  conclude  from  these 
facts  that  the  fluids  are  driven  towards  the  extremities  of 
the  cylinder,  whilst  the  central  portion  remains  in  a  neutral 
state. 

If  a  stick  of  resin  be  rubbed  with  silk  and  brought  near 
the  pith  balls  towards  the  electrical  machine,  they  will  be 
repelled,  showing  that  that  end  of  the  cylinder  is  negatively 
electrified.  If  it  is  brought  near  the  pith  balls  at  the  remote 


How  is  an  insulated  body  affected  by  induction  ?    Explain  the  phenomena  in  detail. 


388  POPULAR    PHYSICS. 

extremity  of  the  cylinder,  they  are  attracted,  showing  that 
that  end  of  the  cylinder  is  positively  electrified.  Finally, 
the  electricities  in  the  two  ends  are  equal  in  quantity,  as 
may  be  shown  by  removing  the  cylinder,  when  they  flow 
together  and  neutralize  each  other. 

The  positive  electricity  of  the  machine,  then,  simply  acts  to  sepa 
rate  the  two  fluids,  attracting  the  negative  fluid  to  the  end  nearest 
it  and  repelling  the  positive  fluid  to  the  opposite  end  of  the  cylinder. 
No  electricity  passes  from  the  machine  to  the  cylinder. 

If,  whilst  the  apparatus  is  in  the  position  shown  in  the 
figure,  the  two  electricities  being  separated,  the  positive 
end  be  touched  by  a  conductor,  as  the  finger,  for  example, 
all  of  the  positive  fluid  will  escape,  whilst  the  negative  fluid, 
being  held  by  the  attraction  of  the  positive  fluid  in  the 
machine,  remains  on  the  surface  of  the  cylinder.  The  cylin- 
inder  is  thus  charged  with  negative  electricity  throughout, 
as  may  be  shown  by  applying  a  rod  of  electrified  glass  or 
resin  to  the  pith  balls  at  the  two  extremities.  Furthermore, 
it  is  immaterial  whereabouts  the  cylinder  is  touched  by  the 
conductor,  for  if  touched  at  any  point,  the  positive  fluid 
escapes,  and  the  cylinder  is  charged  with  the  negative  fluid. 

Had  the  inducing  body  been  charged  negatively,  the 
cylinder  would  in  like  manner  have  received  a  positive 
charge  by  induction. 

The  method  of  induction  is  of  frequent  application  in  experimental 
inquiries,  and  the  principle  set  forth  above  serves  to  explain  a  great 
variety  of  electrical  phenomena. 

The   Electrical  Machine. 

37O.  The  ELECTRICAL  MACHINE  is  a  machine  by  means 
of  which  an  unlimited  amount  of  electricity  'may  be  gen 
erated  by  friction. 

How  does  the  positive,  electricity  act?  How  is  the  negative  fluid  drawn  off? 
Had  the  inducing  body  been  negatively  electrified,  what  would  have  happened? 
(  3 TO.)  What  is  an  Electrical  Machine? 


INDUCTION. 


389 


This  machine  was  invented  about  two  hundred  years  ago  by 
OTTO  VON  GUERICKE.  the  distinguished  inventor  of  the  air-pump. 
The  first  machine  was  simply  a  ball  of  sulphur  fixed  upon  a  wooden 
axis.  On  turning  the  axis,  and  at  the  same  time  pressing  one  hand 


Fig.  254. 


against  the  ball,  a  quantity  of  frictional  electricity  was  developed. 
After  various  improvements  the  machine  has  taken  the  form  shown 
in  Fig.  254.  which  is  the  form  that  is  now  most  generally  employed  in 
physical  researches. 


When  invented  and  by  whom  ? 


390  POPULAR     PHYSICS. 

The  principal  piece  of  the  machine  is  a  plate  of  glass, 
three  feet  or  more  in  diameter.  This  plate  is  mounted  upon 
a  horizontal  axis,  and  may  be  turned  upon  this  axis  by  means 
of  a  crank.  The  wooden  frame  which  supports  the  axis 
embraces  the  plate,  and  bears  four  cushions  which  press 
against  the  glass  on  its  opposite  faces,  two  above  and  two 
below  the  axis.  The  cushions  are  of  leather  stuffed  with 
hair ;  by  their  friction  they  give  rise  to  positive  electricity 
in  the  plate  when  it  is  turned. 

Two  cylinders  of  brass,  AA,  are  mounted  on  the  table 
which  supports  the  frame-work,  and  are  insulated  by  glass 
pillars.  These  cylinders,  called  conductors,  are  united  at 
their  remote  ends  by  a  third  cylinder  of  brass,  as  shown  in 
the  figure.  At  their  ends  nearest  the  plate  they  terminate 
in  cylindrical  pieces  constructed  so  as  to  partially  embrace 
the  plate  but  not  touch  it.  These  pieces  are  called  combs, 
from  the  fact  that  a  great  number  of  projecting  teeth  are 
placed  on  their  sides  next  the  plate.  Finally,  all  of  the 
ends  of  the  cylinders  in  the  machine  are  wrought  into 
spherical  forms,  to  prevent  the  dissipation  of  electricity  as 
much  as  possible.  The  entire  collection  of  metallic  cylinders 
is  called  the  prime  conductor. 

Use   of  the  Electrical  Machine. 

371.  When  the  plate  is  turned  rapidly,  the  friction  of 
the  cushions  or  rubbers  develops  a  great  quantity  of  posi 
tive  electricity  on  the  glass,  whilst  the  negative  fluid  goes 
to  the  rubbers  and  is  conveyed  through  the  frame  to  the 
earth,  and  thus  disappears.  The  neutral  fluid  on  the  con 
ductors  is  decomposed ;  the  negative  fluid  flows  through 
the  teeth  of  the  combs  to  the  glass  plate,  tending  to  neu 
tralize  the  positive  fluid  on  the  plate.  The  conductors  thus 

What  is  the  principal  piece  ?  How  mounted  ?  Describe  the  cushions.  Describe 
the  conductors  and  the  method  in  which  they  are  mounted.  How  are  they  electri 
fied?  What  is  the  prime  conductor  ?  (371.)  Explain  the  operation  of  the  machine. 


INDUCTION.  39 1. 

losing  the   negative   fluid,  become   charged  with    positive 
electricity. 

The  electrical  machine  may  be  arranged  to  produce  negative 
electricity  as  follows  :  The  feet  of  the  table  are  insulated  by  being 
placed  upon  glass  supports,  and  the  prime  conductor  is  then  connected 
with  the  earth  by  a  metallic  chain.  This  chain  permits  the  posithe 
•electricity  to  flow  out  of  the  prime  conductor,  whilst  the  negati\e 
electricity,  being  unable  to  escape,  accumulates  upon  the  cushions, 
table,  and  frame  of  the  instrument. 

Measure  of  the  Quantity  of  Electricity  in  the  Machine. 

372.  The  quantity  of  electricity  in  the  prime  conductor 
may  be  shown  by  an  instrument  called  HENLEY'S  electro 
meter. 

This  electrometer  is  represented  on  the  left  of  the  drawing 
of  the  machine,  and  consists  of  a  vertical  support  of  wood, 
bearing  a  quadrant  divided  into  degrees.  At  the  centre  of 
the  quadrant  is  attached  a  small  arm  of  whalebone,  turning 
around  an  axis  and  terminating  in  a  pith  ball. 

When  the  machine  is  in  operation,  the  ball  rises  along  the 
quadrant,  and  by  its  divergence  from  the  vertical  indicates 
the  quantity  of  electricity  developed. 

Precautions  in  using  the  Machine. 

373.  After  the  prime  conductor  is  electrified,  if  we  cease  to  turn 
the  plate,  and  the  air  is  dry,  the  pith  ball  will  descend  slowly,  show 
ing  a  gradual  dispersion  of  the  electricity.     If  the  air  is  damp,  the 
ball    descends    rapidly,    showing  a  rapid  loss  of  electricity.      Elcc- 
Irical   experiments  seldom  succeed   in  a  damp  day.     In  order  that 
they  should  be  successful,  the  instrument,  as  well  as  the  surround 
ing  atmosphere,  ought  to  be  perfectly  dry. 

Electricity  will  be  developed   more  rapidly  if  the  cushions  are 

How  may  it  be  arranged  to  collect  negative  electricity  f  (372.)  How  is  the 
quantity  of  electricity  on  the  prime  conductor  indicated?  Describe  the  electrometer 
used.  Its  action  (373.)  What  effect  has  dampness  on  the  development  of  elec 
tricity  f  How  is  the  electricity  increased  f 


D^  POPULAR     PHYSICS. 

covered  with  a  paste  composed  of  sulphur  and  tin.  or  an  amalgam 
of  zinc  and  mercury,  or  of  tin  and  mercury. 

Only  a  certain  amount  of  electricity  can  be  retained  on  the  prime 
conductor,  after  which,  if  the  plate  is  turned,  the  tension  becomes  so 
great  that  it  escapes  through  the  air  or  along  the  glass  legs  of  the 
conductor,  and  all  that  is  generated  continues  thenceforth  to  be  dissi 
pated.  The  electrometer  indicates  that  the  instrument  is  fully 
charged,  by  ceasing  to  rise,  and  remaining  stationary  as  the  plate  is 
turned. 

Finally,  in  order  to  attain  the  best  possible  results,  the  machine 
should  not  be  placed  too  near  the  walls,  or  the  furniture  of  a  room, 
or  any  thing  upon  which  it  can  act  by  induction.  In  particular  all 
angular  objects  should  be  avoided.  The  prime  conductor  tends  to 
abstract  from  surrounding  objects  their  negative  electricity,  and  to 
return  to  its  neutral  condition. 


Fig.  255. 

The  effect  of  neighboring  bodies  may  be  illustrated  by  bringing 
a  metallic  point  near  a  charged  prime  conductor,  as  shown  in 
Fig.  255.  When  the  point  is  at  a  considerable  distance  from  the 
conductor,  the  electrometer  begins  to  fall,  showing  a  loss  of  electricity. 

Jfovi  do  we  knmo  when  the  prime  conductor  is  fully  charged  ?  What  is  the 
effect  of  neighboring  conductors  f  How  illustrated  ? 


INDUCTION. 

This  may  be  explained  by  supposing  negative  electricity  to  flow  from 
the  point  to  the  conductor,  in  accordance  with  what  has  been  shown 
before. 

It  is  sometimes  said  that  the  point  draws  off  the  electricity  from 
the  conductor,  but  this  is  not  the  case  ;  the  point  abstracts  none  of 
the  positive  electricity,  but  gives  to  the  conductor  negative  elec 
tricity,  which  unites  with  the  positive  fluid  to  neutralize  it. 

Electrophorus. 

374.  The  ELECTROPHORUS  is  a  machine  due  to  VOLTA, 
by  means  of  which  we  may  obtain  considerable  quantities  of 
electricity. 


; 

--~^ 

Vis.  256. 


It  consists  of  two  pieces  :  one  a  plate  of  resin  spread  on 

Effect  of  the  point.    (314.)  What  is  an  Electrophonis?     Describe  it.     How  is  it 
used  ? 

17* 


394 


POPULAR     PHYSICS. 


a  table  of  wood,  and  the  other  a  wooden  plate,  covered  with 
tin  foil,  and  provided  with  an  insulating  handle  of  glass. 
It  is  represented  in  Figs.  256,  257,  and  258. 

To  use  this  instrument  we  commence  by  rubbing  the 
resinous  plate  vigorously  with  a  cat's  skin,  as  shown  in 
Fig.  256.  This  develops  negative  electricity  in  the  resh . 
We  then  apply  the  disk,  holding  it  by  its  handle.  The  plat  3 
of  resin  acts  upon  the  disk  by  induction,  drawing  the  posi 
tive  fluid  to  the  tin  foil  on  its  lower  face,  and  repelling  the 
negative  fluid  to  the  foil  on  the  upper  face.  In  this  posi 
tion,  if  the  upper  face  be  touched  with  the  finger,  as  shown 
in  Fig.  257,  the  negative  fluid  will  be  drawn  off'  into  the 
body,  and  the  disk  will  be  charged  with  positive  electricity. 


Fig.  25T. 


Fig.  253. 


If  the  disk  be  raised  from  the  resinous  plate  by  its  handle, 
and  touched  with  the  knuckle,  as  shown  in  Fig.  258,  a  spark 

Explain   its  action. 


IXD-JCTIOX. 


395 


will  pass  which  is  due  to  the  negative  electricity,  passing 
from  the  body  to  the  positively  electrified  plate. 

If  now  we  continue  to  repeat  the  manipulation,  exhibited  in 
Figs.  257  and  258,  a  succession  of  sparks  may  be  obtained  without 
the  necessity  of  rubbing  the  resin  again  with  the  cat's  skin.  *  If  the 
air  is  dry,  the  resin  will  continue  in  an  electrified  state  for  a  very 
long  time. 

The  principle  of  continued  induction  lias  been  applied  in  con 
structing  electrical  machines.  That  of  Holtz,  one  of  the  best,  is 
more  powerful  than  the  one  described  in  Art.  870. 

Gold-leaf  Electrometer. 

375.  The  GOLD-LEAF  ELECTROMETER  is  an  instrument 
invented  by  BENNET,  for  determining  whether  a  body  is 


Fig.  2i/.». 


IJow  may  a  succession  of  sparks  le  obtained?    (375.)  What  is  the  Gold-loaf 
Electrometer? 


396  POPULAlt     PUYSICS. 

electrified,  and  to  show  the  kind  of  electricity  with  which 
it  is  charged. 

It  consists  of  a  glass  bottle,  closed  at  the  top  by  a  cork, 
through  which  passes  a  large  copper  wire.  This  wire  is 
terminated  at  its  top  by  a  copper  ball,  and  has  attached  to 
its  lower  extremity  two  slips  of  gold-leaf.  The  instrument 
is  represented  in  Fig.  259. 

The  cork  and  the  whole  top  of  the  bottle  are  covered  with  a  kind 
of  varnish,  made  by  dissolving  sealing-wax  in  alcohol.  The  varnish 
is  laid  on  with  a  brush,  and  serves  to  make  the  bottle  a  better  non 
conductor.  This  kind  of  varnish  is  often  used  in  electrical  experi 
ments  to  render  glass  non-conducting.  Glass  in  a  dry  state  is  a 
good  non-conductor,  but  it  is  apt  to  condense  moisture  from  the  air 
so  as  to  become  a  conductor.  When  covered  with  any  resinous  var 
nish,  this  trouble  is  removed. 

Method  of  using  the   Gold-leaf  ElectrometeV. 

376.  To  ascertain  whether  a  body  is  electrified,  we 
bring  the  ball  of  the  electrometer  near  it.  If  it  is  electri 
fied,  it  acts  upon  the  ball  arid  its  stem  by  induction,  attract 
ing  the  fluid  of  a  contrary  name  into  the  ball,  and  repelling 
that  of  the  same  name  into  the  gold  leaves,  which,  beino- 
very  light  and  electrified  by  the  same  kind  of  fluid,  will 
diverge.  This  instrument  is  very  sensitive,  showing  the 
slightest  amount  of  electricity. 

To  test  the  kind  of  electricity  in  a  body,  bring  it  near 
the  instrument  and  touch  the  ball  with  the  finger.  This 
will  draw  off  the  electricity  of  the  same  name  and  leave 
thut  of  a  contrary  name  to  that  in  the  body.  Now  let 
a  glass  rod  be  rubbed  with  woollen  cloth,  so  as  to  excite 
positive  electricity,  and  then  let  it  touch  the  ball  of  the 
electrometer.  If  the  leaves  diverge  more,  the  electricity 

Describe  it.     Why  is  the  top  of  tJie  bottle  varnished?     Describe  the  varnish. 
(376.)  How  do  we  ascertain  when  a  body  is  electrified  by  this  instrument  ? 


ELECTRICAL     RECREATIONS. 


397 


in  them  before  was  positive,  and  that  of  the  body  in  ques 
tion  was  consequently  negative.  If,  however,  the  leaves 
approach  each  other,  the  electricity  in  them  before  was 
negative,  and  consequently  that  in  the-  body  experimented 
upon  was  positive. 

This  is  an  exceedingly  delicate  test,  and  one  of  great  practical 
value. 


III.  —  ELECTRICAL        RECREATIONS. 

Electrical   Spark.  -  Electrical   Shock. 

.  An  ELECTRICAL  SPARK  is  a  brilliant  flash  of  light 
which  passes  when  a  conductor  approaches  a  highly-elec 
trified  body. 


Is  this  electrometer  ofm  uch  use  ?    (  3  7  7 .)  What  is  an  Electrical  Spark  ? 


3<JS 


POPULAR    pnrsics. 


The  method  of  drawing  a  spark  from  the  prime  conductor 
is  shown  in  Fig.  260.  The  spark,  when  received  by  the 
human  body,  is  accompanied  by  a  sensation,  called  an 
electrical  shock,  which  may  be  very  painful  and  even 
dangerous. 

The  spark  arises  from  the  combination  of  the  two  contrary  fluids. 
The  positive  fluid  acting  at  a  distance  by  induction,  drives  the  posi- 
tive  fluid  of  the  hand  to  the  earth,  and  the  body  of  the  experimenter 
becomes  negatively  electrified.  When  the  tensions  of  the  positive 
electricity  of  the  machine  and  the  negative  electricity  of  the  body 
overcome  the  resistance  of  the  air:  they  rush  together  wiih  a  sharp 
crack  and  a  bright  light  which  constitutes  the  spark.  Whe:i  the 
electrical  machine  is  powerful,  the  sparks  take  a  zig-zag  course,  like 
lightning  from  a  storm-cloud. 


Fig.  261. 


What  i*  a  shock 


What  is  th  e  ca use  of  the  spa rk  ?    Rzplain  in  detail. 


ELECTRICAL    RECREATIONS. 


399 


The  Electrical  Stool. 

378.  A  spark  may  be  drawn  from  the  human  body  when 
properly  electrified.  For  this  purpose  an  ELECTRICAL  STOOL, 
that  is,  a  stool  insulated  by  means  of  glass  legs,  is  made  use 
of,  as  shown  in  Fig.  261.  A  person  standing  on  the  stool, 
and  taking  hold  of  the  prime  conductor,  becomes,  when  the 
plate  is  turned,  positively  electrified.  If  a  second  person 
now  attempts  to  shake  hands  Avith  the  first,  a  shock  will  be 
experienced,  and  a  spark  will  pass  between  them. 


Fig.  262. 


The  Electrical    Chime. 

379.  The  ELECTRICAL  CHIME  is  a  collection  of  bells  that 
are  made  to  ring  by  means  of  electrical  attractions  an  i 
repulsions. 

It  consists,  in  the  case  shown  in  Fig.  2G2,  of  three  bells 
suspended  from  a  horizontal  bar  of  wood,  m.  The  outer 
bells,  b  and  c,  are  suspended  by  metallic  chains,  and  the 


(378.)  What  is  an  Electrical  Stool?    Its  use? 
Chime?     Describe  it. 


(379.)  What  is  an  Electrical 


400 


POPULAR    PHYSICS. 


middle  one  by  a  silk  cord ;  the  middle  bell,  moreover,  is  coi\ 
nected  with  the  earth  by  means  of  a  metallic  chain.  Be 
tween  the  bells  are  two  balls  of  metal,  suspended  from  the 
bar,  w,  by  a  cord  of  silk.  The  entire  apparatus  is  connected 
with  the  prime  conductor  of  an  electrical  machine,  as  shown 
in  Fig.  262. 

When  the  machine  is  turned,  the  outer  bells  become  posi 
tively  electrified,  and  attract  the  balls,  which  impinge  against 
them,  become  electrified,  and  are  immediately  repelled, 
striking  against  the  middle  bell,  where  they  lose  their  charge, 
and  are  again  attracted  to  the  extreme  bells,  and  again 
repelled.  This  alternate  attraction  and  repulsion  of  the 
balls  keep  up  the  ringing  as  long  as  the  plate  is  turned. 


Explain  the  action  of  the  electrical  chime. 


ELECTRICAL     RECREATIONS. 


401 


The  Electrical  Puppet. 

38O.  The  ELECTRICAL  PUPPET  consists  of  a  little  figure 
which  is  made  to  dance  by  means  of  electrical  attraction  and 

repulsion. 

It  consists  of  a  light  image  made  of  elder  pith,  or  some 
similar  substance,  placed  between  two  metallic  plates,  one 
of  which  is  in  connection  with  the  prime  conductor  of  the 
machine,  and  the  other  with  the  earth  by  means  of  a  chain, 

as  shown  in  Fig.  263. 

When  the  machine  is  turned,  the  upper  plate  is  electrified, 
and  attracts  the  image  to  it.  The  image  is  charged  and 
immediately  repelled  to  the  lower  plate,  where  it  loses  its 
electricity,  and  is  again  attracted  to  the  upper  plate,  and  so 
on,  dancing  up  and  down  as  long  as  the  plate  is  turned. 

The  Electrical  Wheel. 


381.  The  ELECTRICAL 
WHEEL  consists  of  four 
or  more  arms,  bent  in  the 
same  direction,  and  at 
tached  to  a  small  cap, 
which  is  free  to  rotate 
about  a  pivot. 

This  pivot  is  attached 
to  the  prime  conductor, 
or  else  to  a  metallic  sup 
port,  connected  with  the 
conductor.  Fig.  264  re 
presents  such  a  wheel.  It 
is  a  reaction  wheel,  and  is 
made  to  turn  by  the  es- 


(380  )  What  is  the  Electrical  Puppet?    Describe  it,    Explain  its  action.    (  381.) 
"What  is  the  Electrical  Wheel?    Describe  it.    Explain  its  action. 


402 


POl'ULAil     P11YS1C.-3. 


cape  of  electricity  from  the  points.  When  the  machine 
is  turned,  the  prime  conductor  and  the  wheel  become  elec 
trified  ;  the  tension  of  the  electricity  at  the  points  becomes 
very  great,  and  finally  escapes  with  a  force  that  causes  the 
wheel  to  revolve  in  a  direction  indicated  by  the  arrow-head 
that  is,  in  a  direction  contrary  to  that  in  which  the  points 
are  bent.  The  wheel  does  Dot  turn  in  a  vacuum,  which 
shows  that  electricity  escapes  from  points  in  a  vacuum  with 
out  resistance. 

The  Electrical  Egg. 

889.     The  ELECTRICAL  EGG  is  an  egg-shaped  light,  pro- 
duced  by  a  flow  of  electricity  through  a  vacuum. 


Tig-  2G3. 


(382.)  What  is  the  Electrical  Egg? 


ELECTRICAL     RECREATIONS.    ,  403 

The  method  of  exhibiting  this  light,  and  the  apparatus  em 
ployed,  are  shown  in  Fig.  265.  The  apparatus  consists  of  a 
hollow  globe  or  oval  of  glass,  containing  two  small  metallic 
spheres  of  metal  at  some  distance  apart.  The  upper  one 
communicates  with  the  prime  conductor,  and  the  lower  one 
with  the  earth.  The  globe  may  be  deprived  of  its  internal 
air  by  means  of  the  air-pump.  Then,  if  the  electrical  ma 
chine  be  turned,  a  flow  of  electricity  will  take  place  from 
the  machine  to  the  earth  through  the  two  balls,  and  because 
the  balls  are  in  a  vacuum  there  will  be  no  obstruction  to  the 
flow.  If  the  experiment  is  made  in  a  darkened  room,  a 
beautiful  violet-colored  light  will  be  seen  between  the  two 
balls,  of  the  shape  shown  in  the  figure. 

The   Electrical   Square. 

383.  The  ELECTRICAL  SQUARE  consists  of  a  square  plate 
of  glass,  upon  one  surface  of  which  a  thin  strip  of  tin  foil  is 
fastened,  running  backwards  and  forwards  across  the  plate, 
as  shown  by  the  black  line  in  Fig.  266.  One  end  of  this 
strip  of  tin  is  made  to  connect  with  the  prime  conductor  of 
the  electrical  machine,  and  the  other  end  is  made  to  com 
municate  with  the  earth  by  a  chain.  The  square  is  insulated 
by  legs  of  glass. 

When  the  machine  is  turned,  a  current  of  electricity  flows 
through  the  strip  of  tin  from  the  machine  to  the  earth,  and 
no  spark  is  given  out.  If,  however,  the  tin  is  broken  at  any 
point,  there  will  be  a  succession  of  sparks  at  that  point, 
which  will  be  so  close  together  as  to  produce  a  continuous 
light.  If,  now,  the  tinlbe  broken  by  a  penknife  so  that  the 
points  of  rupture  are  arranged  in  a  definite  figure,  as  that 
of  a  flower,  for  instance,  a  continuous  light  will  be  seen  at 
each  of  these  points,  and  the  figure  will  appear  as  if  traced 
upon  the  glass  with  fire.  Any  kind  of  figure  may  be  drawn, 

Explain  the  method  of  exhibiting  it.  (383.)  What  is  the  Electrical  Square? 
pescribe  it.  Explain  its  action. 


POPULAR     PHYSICS. 


Fig.  266. 

or  words  may  be  written  on  the  glass.     The  experiment 
is  more  striking  in  a  darkened  room. 

The   Electrical   Cannon. 

384.     The  ELECTRICAL  CAXXOX  is  a  small  cannon  which 
is  discharged  by  means  of  the  electrical  spark. 

This  cannon  is  used  not  only  as   an  electrical  recreation,  but,  it 
serves  also  to  demonstrate  an  important  scientific  fact,  viz. :  that  the 

(  384.)  What  is  the  Electrical  Cannon  ?     What,  is  UK  use,  ? 


ELECTRICAL     RECREATIONS. 


405 


electric  spark  is  capable  of  producing  chemical  reactions.  For  ex 
ample,  water  is  formed  of  oxygen  and  hydrogen  gases  in  the  propor 
tion  of  two  volumes  of  the  former  to  one  volume  of  the  latter.  Now 
if  these  two  ga.ses  be  mixed  in  this  proportion,  and  an  electrical 
spark  be  passed  through  the  mixture,  the  gases  instantly  unite  and 
form  water.  Moreover,  the  combination  takes  place  with  a  brilliant 
flash  of  light  and  a  loud  report,  the  report  being  due  to  the  expansive, 
force  of  the  vapor  which  is  produced  at  the  moment  of  combination. 
Tt  is  upon  these  principles  that  the  electrical  cannon  represented  in 
Fig.  267  is  constructed. 


It  consists  of  a  small  copper  cannon  mounted  on  a  stem 
of  glass.  In  the  vent  of  the  cannon  is  a  tube  of  glass, 
through  which  passes  a  copper  wire.  This  wire  terminates 
externally  in  a  ball,  and  internally  it  terminates  near  the 
metal  of  the  cannon  without  touching  it.  The  whole  com 
municates  with  the  earth  by  a  chain. 


Illustrate.    Describe  it. 


406 


POPULAR     PHYSICS. 


To  use  the  instrument,  it  is  filled  with  a  mixture  of  oxygen 
and  hydrogen  in  the  proportions  to  form  water,  and  tfio 
muzzle  is  then  closed  by  a  cork.  If  a  chargeci  electrophorus 
3  brought  in  contact  with  the  ball,  a  spark  passes  between 
them,  and  another  between  the  internal  extremity  of  the 
wire  and  the  metal  of  the  cannon.  This  spark  causes  an 
explosion  and  drives  out  the  cork. 

A  similar  apparatus,  called  VOLTA'S  pistol,  is  used  for  explode 
a  mixture  of  oxygen  and  hydrogen,  or  of  air  and  hydrogen!  It  is 
nothing  more  than  a  sheet-iron  cylinder  closed  by  a  cork  It  is 
exploded  by  touching  its  button  to  the  prime  conductor  of  an  electri 
cal  machine. 


IV.  — ACCUMULATION   OF   ELECTRICITY. 

Electrical  Condenser. 

385.  An  ELECTRICAL  CONDENSER  is  an  apparatus  em 
ployed  for  the  accumulation  of  electricity. 

They  are  of  various  forms,  but  are  all  essentially  com 
posed  of  two  conductors  separated  by  an  insulator.  The 
condenser  of  EPINUS  may  serve  as  a  type  of  this  class  of 
apparatus. 

Condenser  of  Epinus. 

386.  The  CONDENSER  OF  EPINUS  is  composed  of  two 
metallic  plates,  A  and  B,  Fig.  268,  standing  upon  supports 
of  glass,  with  an  intervening  plate  of  glass,  (7,  somewhat 
larger  than  either  of  the  metallic  plates.  These  several 
plates  are  so  mounted  that  the  plates .4  and  J3  maybe  made 
to  approach  to,  or  recede  from  the  plate  C. 

How  is  it  used?    Describe  VOLTA'S  pistol.    (385.)  What  is  an  Electrical  Con 
denser?    (  386.)  Describe  the  Condenser  of  EPINUS. 


ACCUMULATION     OF     ELECTRICITY. 


4:07 


Fig.  268. 

Method   of  using  the   Condenser. 

387.     To  use  the  condenser,  the  plates,  A  and  J?,  are 
moved  up  to  touch  the  plate,  C,  as  shown  in  Fig.  269.    The 


Fig.  269 


( 387.)  Describe  the  method  of  using  it",  in  detail. 


403  POPULAR     PHYSICS. 

plate,  B,  is  made  to  communicate  with  the  prime  conductor 
of  an  electrical  machine,  and  the  plate,  A,  with  the  earth. 
The  electrical  machine  is  then  put  in  motion,  which  charges 
the  plate,  B,  with  positive  electricity.  Were  it  not  for  the 
plate,  A,  the  quantity  of  electricity  on  each  unit  of  surface  of 
B  would  be  the  same  as  on  a  unit  of  surface  of  the  prime 
conductor;  but  the  presence  of  the  plate,  A,  modifies  this 
result. 

The  plate,  7>,  acts  by  induction  upon  A,  and  drives  its 
positive  electricity  to  the  earth,  retaining  its  negative 
electricity  by  the  force  of  attraction.  The  negative  elec 
tricity  of  A  now  reacts  upon  B,  partially  neutralizing  the 
effect  of  its  positive  electricity.  The  electricity  of  B  being 
partially  neutralized,  no  longer  holds  that  of  the  prime  con 
ductor  in  equilibrium,  and  an  additional  quantity  of  the 
positive  fluid  flows  into  it,  which,  acting  as  before,  draws 
into  A,  from  the  earth,  an  additional  quantity  of  the  negative 
fluid,  and  so  on.  In  this  way  there  is  gradually  accumulated 
upon  B  and  A,  large  quantities  of  the  positive  and  negative 
fluids. 

When  the  apparatus  is  fully  charged,  we  cut  off  the  com 
munication  between  A  and  the  earth,  then  that  between  B 
and  the  machine,  by  taking  away  the  chains.  In  this  condi 
tion  the  two  electricities  on  A  and  B  show  no  effects,  but 
simply  hold  each  other  in  equilibrium.  There  is,  however, 
in  consequence  of  the  intervening  glass  plate,  an  excess  of 
electricity  in  B,  as  is  shown  by  the  electrical  pendulum,  b, 
placed  in  connection  with  it.  A  similar  pendulum  placed  in 
connection  with  A,  gives  no  such  indication. 

If,  now,  the  plates  be  separated,  as  shown  in  Fig.  268,  both 
electrical  pendulums  will  diverge,  as  they  should  do,  because 
the  two  electricities  no  longer  hold  each  other  in  equilibrium. 
In  this  condition  the  electricities  of  the  two  plates  may  be 


Explain  its  theory.    How  may  it  be  shown  that  the  two  plates  are  differently 
charged  ? 


ACCUMULATION     OF    ELECTRICITY.  409 

tested  and  shown  to  be  opposite.  If  a  rod  of  glass  be 
rubbed  with  silk  and  brought  near  the  pendulum  upon  B,  it 
will  be  repelled,  indicating  positive  electricity;  if  it  be 
brought  near  the  pendulum  upon  A,  it  will  be  attracted, 
indicating  negative  electricity. 

Slow  discharge  of  the  Condenser.— Instantaneous  discharge.— 
Discharger. 

388.  The  condenser  being  charged  and  placed  as  in 
Fig.  269,  may  be  discharged,  that  is,  brought  back  to  its 
neutral  state,  in  two  ways.  First,  by  successive  contacts,  in 
which  case  the  discharge  takes  place  slowly  ;  or  secondly, 
by  connecting  the  plates  A  and  £  by  a  conductor,  in  which 
case  the  discharge  is  instantaneous. 

If  the  plate,  A,  is  touched,  no  electricity  is  drawn  off, 
because  all  that  it  contains  is  held  in  equilibrium  by  that  in 
the  plate,  B.  If,  however,  the  plate,  B,  is  touched,  all  of 
its  free  electricity,  that  is,  all  which  is  not  neutralized  by 
that  in  the  plate,  A,  is  drawn  off.  After  this,  a  certain  un- 
neutralized  portion  of  electricity  will  exist  upon  A,  which 
will  be  indicated  by  the  pendulum.  By  continuing  to  touch 
the  plates  alternately,  the  whole  charge  may  be  drawn  off 
in  small  quantities. 

To  obtain  an  instantaneous  discharge,  we  might  touch  one 
plate  with  one  hand  and  the  other  plate  with  the  other 
hand,  when  the  two  fluids  would  flow  through  the  body  and 
neutralize  each  other  ;  this  method  produces  a  shock  much 
more  powerful  than  that  produced  by  the  simple  spark  from 
the  prime  conductor. 

To  avoid  this  shock  we  make  use  of  a  discharger.  A  dis 
charger  consists  of  a  heavy  wire  bent  into  an  arc,  terminated 
at  its  two  ends  by  balls,  and  having  a  hinge  joint  in  the 

(388.)  In  how  many  \vays  may  a  condense!'  be  discharged?  Describe  the  first 
method.  The  second  method.  Describe  the  method  by  successive  contacts.  How 
may  it  be  instantaneously  discharged  ?  What  is  a  discharger,  and  what  is  its  use? 

18 


410  POPULAK    PHYSICS. 

middle,  so  that  it  can  be  folded  upon  itself,  as  shown  in 
Fig.  271.  It  is  usually  provided  with  a  glass  handle,  by 
which  it  is  held. 

To  discharge  the  condenser,  one  ball  is  brought  in  contact  with 
one  plate,  and  being  held  there,  the  discharger  is  folded  so  that  the 
o'her  ball  will  touch  the  second  plate.  At  the  instant  of  contact  a 
spark  is  emitted,  arising  from  the  combination  of  the  two  fluids,  which 
takes  place  through  the  discharger.  No  shock  is  felt,  because  the 
electricity  does  not  pass  through  the  body  as  in  the  previous  case, 

Limit  of  the  Charge  in  a  Condenser. 

3 §9.  Two  circumstances  limit  the  amount  of  electricity 
that  may  be  accumulated  in  a  condenser.  First,  the  un 
balanced  electricity  in  the  plate,  _Z?,  goes  on  augmenting 
with  the  charge,  until  at  last  its  tension  becomes  equal  to 
that  on  the  prime  conductor,  after  which  no  more  can  flow 
into  the  condenser  from  the  machine.  Secondly,  the  two 
electricities  on  the  plates,  A  and  J3,  tend  to  unite  with  an 
energy  which  goes  on  augmenting  with  the  accumulation 
of  electricity  on  the  plates,  and  may  ultimately  become  so 
great  as  to  break  through  the  glass  and  thus  cause  a  union 
of  the  two  fluids. 

The  Leyden  Jar. 

39O.  The  LEYDEN  JAR,  named  from  the  city  where  it 
was  invented,  is  a  condenser,  differing  only  in  form  from 
that  which  has  been  described. 

In  its  improved  form  it  consists  of  a  bottle  or  jar  of  thin 
glass,  as  shown  in  Fig.  270,  nearly  covered  on  its  outside 
with  tin  foil,  and  nearly  filled  within  by  loose  tin  foil,  or  some 
other  metallic  substance  in  a  loose  state.  A  wire  passing 
through  the  cork  extends  to  the  metallic  filling  within,  and 
terminates  externally  in  a  sphere  of  metal  called  the  button. 


ffow  is  it  employed?     (389.)  What  circumstances  limit  the  charge  in  a  con- 
denser?    (  390.)  What  is  the  Leyden  Jar?    Describe  it, 


ACCUMULATION     OF     ELECTKICITY.  4:11 

This  is  a  condenser  in  which  the  glass  of  the  bottle  serves  as  an 
insulator,  whilst  the  metallic  substances  within  and  without  corre 
spond  to  the  metallic  plates  in  the  instrument  already  described. 
The  interior  metal  corresponds  with  the  plate,  B,  and  may  be  called 
the  collector,  whilst  the  external  metal  corresponds  with  the  plate,  A. 
What  has  been  said  of  the  condenser  holds  good  for  the  Leyden  jar. 


Fig.  270. 

The  Leyden  jar  is  charged  by  holding  the  outer  tinned 
part  in  the  hand,  and  bringing  the  button  in  contact  with 
the  prime  conductor  of  an  electrical  machine,  as  shown  in 
Fig.  270.  The  positive  fluid  is  accumulated  in  the  interior, 
and  acts  by  induction  upon  the  outer  coating,  which  becomes 
negative,  the  positive  fluid  in  that  coating  being  conveyed 
away  by  the  hand  through  the  body.  As  in  the  condenser, 
the  two  fluids  react  so  as  to  accumulate  a  large  quantity  of 
positive  electricity  on  the  inside  of  the  jar,  and  of  negative 
electricity  on  the  outside. 

After  the  jar  has  been  charged,  if  it  be  held  in  one  hand  whilst 
the  other  is  brought  in  contact  with  the  button,  a  shock  will  be  felt 
through  the  arms  and  body,  and  the  jar  will  return  to  its  neutral 
state.  When  it  is  desirable  to  discharge  the  jar  without  the  shock. 


How  (Joes  it  resemble  a  condenser?     How  is  the  jar  charged?     Explain  the 
theory.     TIow  is  it  discharged  ? 


412 


POPULAR     PHYSICS. 


the  discharger  is  used,  as  shown  in  Fig.  271.     One  ball  of  the  dis 
charger  is  made  to  touch  the  outer  coating,  and  the  other  is  then 


Fig.  271. 

brought  in  contact  with  the   button.     In  this  case  there  is  a  spark 
emitted,  and  the  jar  returns  to  its  neutral  condition. 

Electrical  Battery. 

391.  An  ELECTRICAL  BATTERY  consists  of  an  assemblage 
of  Ley  den  jars  so  connected  as  to  act  like  a  single  condenser, 
as  shown  in  Fig.  272. 

The  jars  are  placed  in  a  box  whose  bottom  is  lined  with 
metal,  which  serves  to  connect  their  outside  surfaces.  Their 
inside  surfaces  are  brought  into  communication  by  connect 
ing  the  several  buttons  with  metallic  rods. 

Tn  batteries  the  jars  are  made  large,  and  are  covered  within  and 
without  with  tin  foil,  the  interior  lining  being  brought  into  commu 
nication  with  the  button  of  each  jar  by  a  metallic  chain.  Upon  one 


(  391 .)  What  is  an  Electrical  Battery  ?    Describe  it. 
in  latteries  ? 


What  kind  of  jars  are  used 


ACCUMULATION     OF    ELECTRICITY. 


413 


of  the  buttons  is  placed  an  electrical  pendulum,  which  indicates  the 
excess  of  the  fluid  on  the  inner  over  that  on  the  outer  surface. 

The  battery  is  charged  in  the  same  manner  as  the  condenser  of 
EPINUS  (Art.  391).  When  charged,  the  chains  are  removed  by  a 
hook  with  a  glass  handle. 


Fig.  272. 

In  discharging  an  electrical  battery,  a  discharger  is  used  with  two 
glass  handles,  as  shown  in  Fig.  276.  Care  should  be  taken  to  touch 
the  external  covering  before  touching  the  common  button  with  the 
discharger. 

Condensing  Electrometer. 

392.  The  gold-leaf  electrometer,  shown  in  Fig.  259,  is 
very  sensitive,  but  it  may  be  rendered  still  more  so  by  the 
addition  of  two  disks  or  condensing  plates,  as  shown  in 
Figs.  273  and  274.  The  inferior  disk  is  attached  perma 
nently  to  the  stem,  £,  which  supports  the  strips  of  gold-leaf. 
The  superior  disk  has  a  glass  handle  by  which  it  can  be 
removed  at  pleasure.  The  two  disks  are  of  brass,  coated 

How  connected?  How  charged?  Uow  discharged?  ( 392.)  What  is  a  Con 
densing  Electrometer  ? 


414 


POPULAR     PHYSICS. 


with  varnish,  which  serves  as  an  insulator,  taking  the  place 
of  the  glass   plates   in   the   condensers   already  described, 


Fig.  273. 


Fig.  274. 


but  bfiiog  very  thin,  the  condensing  power  is  much  aug 
mented. 

Use   of  the  Condensing  Electrometer. 

393.  To  use  the  condensing  electrometer  for  detecting 
small  quantities  of  electricity,  we  place  the  upper  disk  upon 
the  lower  one,  as  shown  in  Fig.  273,  then  using  the  lower 
disk  as  a  collector  we  bring  it  into  contact  with  the  body 
to  be  experimented  upon.  At  the  same  time  we  establish 
a  connection  between  the  upper  disk  and  the  earth,  by 
touching  it  with  the  finger. 

(  393.)  How  is  the  condensing  electrometer  used? 


EFFECTS     OF     ACCUMULATED     ELECTRICITY.  415 

In  Fig.  273.  the  body  experimented  upon  consists  of  two  plates, 
one  of  zinc,  and  the  other  of  copper,  fastened  together.  We  shall 
see  hereafter,  that  by  a  simple  contact  of  two  such  plates,  the  zinc 
is  positively,  and  the  copper  negatively  electrified.  This  last  metal 
then  being  brought  into  contact  with  the  inferior  plate,  yields  its 
negative  electricity,  which  acting  by  induction  through  the  varnish, 
renders  the  upper  plate  positive.  When  the  two  electricities  hn\ ) 
accumulated  upon  the  plates,  we  first  withdraw  the  finger,  and  then 
the  plate  cz.  If  the  upper  plate  be  lifted  off,  the  negative  electricity 
which  was  before  held  in  equilibrium,  becomes  free,  and  the  gold 
leaves  diverge,  as  shown  in  Fig.  274. 

In  this  manner  quantities  of  electricity  may  be  discovered  so  small 
as  to  be  unnoticed  by  the  simple  electrometer. 


V. EFFECTS       OF       ACCUMULATED       ELECTRICITY. 

Physiological   Effects  of  Electricity. 

394.  The  physiological  effects  of  electricity  are  the 
effects  which  it  produces  on  men  and  animals.  They  con 
sist  of  muscular  contractions,  accompanied  by  a  greater  or 
less  amount  of  pain,  according  to  the  power  of  the  electrical 
apparatus. 

When  we  receive  a  simple  spark  from  the  prime  conductor,  we 
experience  only  a  slight  stinging  sensation:  with  a  small  Leyden 
jar,  the  pain  is  felt  extending  up  the  arms  to  the  elbows  or  shoulders ; 
with  a  more  powerful  jar  or  a  battery  the  shock  is  felt  through  the 
arms  and  chest,  and  may  be  sufficient  to  produce  death. 

An  electric  shock  may  be  given  to  a  great  number  of  persons  at 
the  same  time.  To  that  end  they  form  a  chain  by  taking  each  other 
by  the  hand,  as  shown  in  Fig.  275;  then  the  person  at  one  end 
takes  a  Leyden  jar  in  his  hand  :  the  circuit  is  completed  by  the 
person  at  the  other  end  of  the  chain  touching  the  button  of  the  jar. 
when  the  shock  is  felt  simultaneously  throughout  the  ring.  NOLLET 


Explain  Fig  273.  What  are  the  advantage*  of  this  instrument  ?  (  394.)  What 
are  the  physiological  effects  of  electricity  ?  Describe  the  shock.  How  may  the  shock 
be  givan  to  a  number  of  persons? 


4:16 


POPULAR   PHYSICS. 


administered  in  this  manner,  in  the  presence  of  Louis  XV.,  an  elec« 
trical  shock  to  an  entire  regiment  of  soldiers. 


Fig.  275. 

With  a  battery,  tne  shock  becomes  so  powerful  as  to  render  it 
dangerous  to  attempt  receiving  it.  With  a  battery  of  only  six  jars 
of  mean  size,  it  would  be  hazardous  to  receive  the  shock.  With 
more  powerful  batteries,  cats,  dogs,  and  even  stronger  animals  may 
be  killed  by  a  single  shock.  Fig.  276  represents  a  dog  killed  by  a 
shock  from  a  battery  of  nine  jars.  The  metallic  collar  of  the  dog  is 
connected  with  the  exterior  coating  of  the  battery,  then  one  ball  of 
the  discharger  is  placed  near  the  posterior  part  of  the  dog's  spinal 
column,  after  which  the  circuit  is  completed  by  touching  the  button 


Examples.     What  is  the  effect  of  a  shock  from  a  powerful  "battery?    Explain 
the  experiment. 


EFFECTS     OF    ACCUMULATED    ELECTRICITY.  417 

of  the  battery  with  the  other  ball  of  the  discharger.     The  animal  is 
killed  instantly. 

In  the  Museum  at  Harlem,  in  Holland,  is  a  battery  whose  dis 
charge  is  capable  of  killing  an  ox.     There  is  also  a  very  powerful 


Fig.  276. 

battery  in  the  Conservatory  at  Paris,  which  was  given  to  it  by  the 
Physicist  CHARLES. 

Heating  Power  of  Electricity. 

395.  The  heat  developed  by  electricity  is  sufficient  not 
only  to  inflame  ether,  gunpowder,  and  the  like,  but  also  to 
melt  and  volatilize  the  metals. 

Fig.  277  represents  the  manner  of  inflaming  ether.  The 
ether  is  poured  into  a  glass  vase,  through  the  bottom  of 


The,   Harlem  battery.    (395.)  Is  there  much  heat  developed  by  electricity? 
Explain  the  experiment  shown  in  Fig.  2771 


418 


POPULAR     PHYSICS. 


which  passes  a  metallic  wire,  terminating  in  a  button.  The 
wire  is  connected  by  a  chain  with  the  outer  covering  of  a 
Ley  den  jar.  When 
the  circuit  is  com 
pleted  by  touching 
the  button  of  the  ap 
paratus  with  that  of 
the  jar,  a  spark  is 
given  off,  and  heat 
enough  developed  to 
inflame  the  ether. 

This  experiment  suc 
ceeds  with  a  very  small 
jar,  or  even  a  simple 
spark  from  the  prime 
conductor.  The  experi 
ment  may  be  made  more 
interesting  by  standing 
upon  the  electrical  stool 
(Fig.  261),  and  inflam 
ing  the  ether  with  the 
finger.  The  ether  may 
be  inflamed  by  a  spark 
from  a  piece  of  ice  held 
in  the  hand. 


Fig.  277. 


When  an  electrical  battery  is  discharged  through  a  fine 
metallic  wire,  it  may  be  melted  or  even  volatilized,  according 
to  the  power  of  the  battery. 

In  performing  this  experiment  it  will  be  best  to  use  the  universal 
discharger.  This  instrument  and  the  manner  of  using  it.  are  shown 
in  Fig.  278.  The  discharger  consists  of  two  copper  wires,  A  and  Ji} 
mounted  upon  glass  supports.  The  wires  can  slide  freely  through 
the  rings  that  hold  them,  and  can  furthermore  be  turned  about  hinge 


What  variation  may  be,  made,  in  it  ?    How  may  a  wire  be  melted?    Explain  the 
construction  and  use  of  the  universal  discharger. 


EFFECTS     OF     ACCUMULATED     ELECTRICITY. 


419 


joints,  so  as  to  bring  their  buttons  as  near  as  may  be  desired  to  any 
body  that  is  placed  upon  the  stand,  M. 

To  melt  a  wire  by  electricity,  we  attach  it  to  the  two  inner  but 
tons  at  t,  then  connect  one  of  the  wires.  A,  for  example,  with  the 
exterior  coating  of  the  battery,  and  complete  the  circuit  by  connect 
ing  B  with  the  button  of  one  of  the  jars  of  the  battery.  This  is 


Fig.  278. 

effected  in  the  manner  shown  in  the  figure,  the  connecting  chain 
being  managed  by  means  of  a  hook  with  a  glass  handle.  At  the 
instant  of  contact,  the  wire,  if  fine  enough,  is  melted  into  globules 
and  even  volatilized,  that  is.  reduced  to  vapor,  which  disappears  in 
the  air. 

When  the  wire  is  a  little  larger,  it  simply  becomes  red  hot  and 
gives  forth  a  brilliant  lisht  ;  if  still  larger,  it  becomes  heated  with 
out  being  luminous.  Fine  and  short  wires  may  be  melted  under 


Explain  the  experiment  ofmeUiriy  a  wife  in  detail. 


420 


POPULAR     PHYSICS. 


water  in  the  same  manner  as   in  air.  but  the  experiment  is  more 
difficult  to  make. 

Mechanical   Effects   of  Electricity. 

396.  The  MECHANICAL  EFFECTS  OF  ELECTRICITY  are 
manifested  when  large  charges  of  electricity  are  passed 
through  imperfect  conductors.  They  consist  of  violent 
expansions,  with  tearing,  fracturing,  and  the  like. 

These  effects  are  generally  exhibited  by  placing  the  body  upon  the 
plate,  M,  of  the  universal  discharger  (Fig.  278),  and  then  passing  a 


Fig.  279. 

powerful  charge  from   a  battery  through  it.     In  this  way  a  small 
block  of  wood  may  be  torn  to  splinters  in  an  instant. 

Fig.  279  represents  an  apparatus  by  means  of  which  a  hole  may 
be  torn  in  a  card  by  using  a  single  Leyden  jar.  A  can}  is  placed  at 
the  top  of  a  glass  cylinder,  beneath  which  is  a  wire  projecting  from 

(  396.)  What  are  some  of  the  mechanical  effects  of  electricity  ?  ffow  exhibited ) 
Explain  the  method  of  perforating  a  card  by  electricity. 


ATMOSPHERIC     ELECTRICITY.  4:21 

a  metallic  plate.  The  plate  connects  by  a  chain  with  the  exterior 
coating  of  the  jar.  Above  the  card  is  a  second  wire  which  is  insu 
lated  in  the  manner  shown  in  the  figure.  When  the  circuit  is  com 
pleted,  by  touching  the  upper  wire  with  the  button  of  the  jar,  a  shock 
follows,  and  the  card  is  found  to  have  been  pierced  as  if  run  through 
by  a  needle  or  pin. 

To  pierce  a  plate  of  glass  requires  a  large  battery.  The  battery 
belonging  to  Harlem  Museum  (Art.  394).  is  capable  of  piercing  a 
book  of  four  hundred  pages. 

A  partial  account  of  the  chemical  effects  of  electricity  has  been 
given  in  speaking  of  the  electrical  cannon.  More  on  this  subject  will 
be  given  when  we  come  to  treat  of  the  effects  of  the  Voltaic  pile. 


VI,  —  ATMOSPHERIC      ELECTRICITY. 

Identity  of  Lightning  and  the  Electrical  Spark. 

397.  FRANKLIN  published  a  memoir  in  1749,  showing 
the  complete  parallelism  between  the  electricity  of  the 
clouds  and  that  of  the  electrical  machine.  In  that  memoir 
he  suggested  that  the  electricity  of  the  clouds  might  be 
attracted  to  the  earth  by  means  of  points,  and  recommended 
that  the  experiment  should  be  made. 

In  accordance  with  that  suggestion,  the  experiment  was 
first  made  by  DALIBARD,  in  May,  1752.  He  erected  in  his 
garden  a  rod  of  iron  about  forty  feet  high,  having  its  upper 
extremity  terminating  in  a  point.  After  the  passage  of  a 
thunder  cloud,  the  rod  was  found  to  be  electrified,  and  for 
the  space  of  fifteen  minutes  sparks  were  drawn  from  it,  which 
were  used  in  charging  several  Leyden  jars. 

About  a  month  later,  FRANKLIN,  without  any  knowledge 
of  the  discovery  of  DALIBAKD;  succeeded  in  attracting 
electricity  from  a  cloud  to  the  earth.  He  raised  a  silken 
kite,  just  before  a  coming  thunder  storm.  The  string  of 

Of  piercing  a  plate  of  glass.  ( 397.)  Who  first  showed  the  identity  of  lightning 
and  electricity?  Explain  DALIBAKD'S  experiments.  Explain  FRANKLIN'S  experi 
ments. 


422  POPULAR    PHYSICS. 

the  kite  was  of  hemp  ;  attached  to  the  lower  end  of  it  was 
a  small  key,  and  fastened  to  the  key  was  a  silken  cord,  by 
which  the  kite  might  be  insulated.  It  was  only  after  the 
string  became  damp  from  the  falling  rain  that  the  key 
showed  signs  of  being  electrified.  He  was  at  last  rewarded 
by  obtaining  an  electric  spark.  So  great  was  his  joy  that 
he  could  not  refrain  from  bursting  into  tears. 

The  complete  identity  between  lightning  and  the  electric 
spark  was  thus  established,  and  all,  even  DALIBARD  himself, 
unite  in  attributing  to  FRANKLIN  the  honor  of  the  dis 
covery. 

Atmospheric   Electricity. 

398.  The  existence  of  atmospheric  electricity  is  not  con 
fined  to  clouds  alone,  for  it  often  exists  in  the  atmosphere 
when  no  trace  of  a  cloud  is  visible.  In  this  case  the  elec 
tricity  is  positive.  It  is  most  abundant  in  open  spaces  and 
at  considerable  elevations.  In  houses,  in  the  streets,  under 
trees,  and  in  sheltered  localities,  no  trace  of  free  electricity 
is  discoverable.  During  storms  the  electricity  of  the  air  is 
sometimes  positive  and  sometimes  negative.  All  clouds  are 
supposed  to  be  electrified,  some  positively  and  some  nega 
tively. 

The  electrical  condition  of  clouds  may  be  determined  by 
metallic  rods,  by  kites,  or  by  small  balloons  held  by  a  string 
in  the  hand. 

The  electrical  state  of  the  atmosphere  may  be  determined  in  a 
great  variety  of  ways.  M.  BECQUEREL  employed  for  this  purpose 
the  gold-leaf  electrometer  shown  in  Fig.  259.  Instead  of  the  button 
he  used  a  stem  of  metal,  attaching  to  its  upper  end  a  fine  and  flexible 
wire.  To  the  second  extremity  of  the  wire  he  fastened  an  arrow, 


( 398.)  What  is  the  nature  of  the  electricity  of  the  air  ?  "Where  is  it  most  abund 
ant  ?  What  is  the  state  of  the  atmosphere  during  storms  ?  How  is  the  electrical 
condition  of  the  clouda  determined  ?  How  is  the  electrical  state  of  the  atmosphere 
determined  ? 


ATMOSPHERIC    ELECTKICITY.  423 

which,  being  shot  from  a  bow,  ascended  into  the  atmosphere,  draw 
ing  the  wire  with  it.  When  the  arrow  was  shot  directly  upwards, 
the  divergence  of  the  gold  leaves  indicated  the  existence  of  free 
electricity,  and  the  nature  of  this  electricity  was  tested  as  already 
explained. 

Lightning  and  Thunder. 

399.  LIGHTNING  is  nothing  else  than  an  elongated 
electrical  spark,  which  passes  between  two  differently  elec 
trified  clouds  when  brought  near  each  other.  Sometimes 
a  discharge  takes  place  between  a  cloud  and  the  earth  ;  this 
is  called  a  thunderbolt. 

A  flash  of  lightning  is  often  of  great  length,  and  as  it  takes  place 
along  the  line  of  least  resistance,  it  generally  follows  a  zig-zag  path, 
as  is  often  the  case  with  the  spark  from  a  Leyden  jar.  When  a  flash 
of  lightning  is  seen  in  the  lower  regions  of  the  atmosphere,  it  has  a 
brilliant  white  color;  but  in  the  higher  regions,  where  the  air  is 
rarefied,  it  assumes  a  violet  hue,  similar  to  that  of  the  electric  eg£ 
(Art.  382). 

THUNDER  is  the  sound  which  follows  a  flash  of  lightning. 
It  is  due  to  vibrations  caused  by  the  passage  of  the  spark 
through  the  air. 

Thunder  is  not  heard  till  an  appreciable  time  after  the  flash  is 
perceived.  This  arises  from  the  fact  that  light  travels  with  im 
mense  velocity,  reaching  the  eye  instantaneously,  whilst  sound 
travels  more  slowly,  and  reaches  the  ear  only  after  a  sensible  inter 
val  of  time.  The  distance  of  a  clap  of  thunder  may  be  ascertained 
by  counting  the  number  of  seconds  between  the  flash  and  the  report, 
and  allowing  five  seconds  to  a  mile. 

The  intensity  of  the  sound  diminishes  as  the  distarc'i  becomes 
greater  :  near  by,  it  is  sharp  and  rattling,  like  boards  falling  one 


(399.)  What  is  Lightning?  What  is  a  thunderbolt  ?  Why  if  the  flash  often  zig. 
sag  in  its  shape  ?  What  is  the  color  of  the  flakh  ?  What  is  Thunder  ?  Why  is  the 
thunder  only  heard  after  an  appreciable  time?  How  may  the,  dixtanee  of  the 
flash  be  determined  ?  What  effect  has  distance  on  the  Sound  of  thunder  ? 


424:  POPULAR     PHYSICS. 

upon  the  other ;    at  a  greater  distance,  it  is  dull  and  prolonged  in  a 
low  rumble  of  varying  intensity. 

The  rattling  or  rolling  of  thunder  is  differently  explained.  By 
some  it  is  said  to  be  due  to  a  succession  of  echoes  from  the  clouds 
and  the  earth.  Others  regard  lightning,  not  as  a  single  spark,  but  as 
a  succession  of  sparks,  each  giving  rise  to  separate  explosions  that 
succeed  each  other  so  rapidly  as  to  produce  a  continuous  rumbling 
sound.  Others  again  attribute  the  rolling  of  thunder  to  the  zig-zag 
course  of  the  lightning,  the  sound  from  different  points  of  the  zig-zag 
path  reaching  the  ear  in  times  proportional  to  their  distances.  In 
this  way  the  sounds  from  different  points  are  superposed  irregularly, 
giving  rise  to  irregularity  in  the  resulting  sound. 

The  Thunderbolt. 

1OO.  A  THUNDERBOLT  is  a  discharge  of  electricity  be 
tween  a  cloud  and  the  earth. 

When  an  electrified  cloud  passes  near  the  earth,  it  acts  upon  it  by 
induction,  repelling  the  fluid  of  the  same  name  and  attracting  that 
of  an  opposite  name.  As  soon  as  the  tension  of  the  two  electricities 
becomes  greater  than  the  resistance  of  the  intervening  air,  a  spark  or 
flash  passes,  and  the  thunderbolt  is  said  to  fall,. or  the  lightning  to 
strike.  The  flash  generally  passes  from  the  cloud  to  the  earth,  but 
sometimes  the  reverse  is  the  case.  The  attraction  between  the  two 
electricities  increases  as  the  distance  diminishes.  Hence  it  is  that 
elevated  objects  are  most  likely  to  be  struck,  such  as  spires,  high 
trees,  lofty  buildings,  and  the  like.  Good  conductors,  like  metals, 
moist  bodies,  trees,  and  the  like,  are  more  likely  to  be  struck  than 
bad  conductors.  Hence  the  danger  of  taking  refuge  under  a  tree  in 
a  thunder  storm. 

Effects   of  the  Thunderbolt. 

4O1.  The  effects  of  the  thunderbolt  are  extremely  various  and 
wonderful.  It  crushes  or  fractures  bad  conductors,  inflames  com- 


How  is  the  rattle  or  roll  of  thunder  accounted  for  ?  ( 400.)  What  is  a  Thunder 
bolt  ?  Why  does  lightning  strike  ?  Explain  the  phenomenon.  What  bodies  are 
most  likely  to  be  struck?  What  least  likely?  (4O1.)  Describe  the  effects  of  the 
thunderbolt. 


ATMOSPHERIC    ELECTRICITY.  425 

bustible.  bodies,  melts  metals,  reverses  the  poles  of  magnets,  and 
often  kills  men  and  animals.  Sometimes  it  falls  slowly  in  the  form 
of  a  globe  of  fire,  and  then  explodes  with  a  noise  like  a  battery  of 
cannon.  It  is  this  form  of  lightning  that  is  most  likely  to  inflame 
the  edifices  which  it  chances  to  strike. 

It  is  said  that  a  ball  of  electrical  fire  fell,  in  1718,  near  Brest, 
striking  a  house  with  such  force  that  the  roof  sprung  up  as  if  a  mine 
had  been  exploded  beneath  it.  and  the  stones  of  the  walls  were 
scattered  in  all  directions,  some  being  carried  to  the  distance  of  a 
hundred  and  fifty  feet. 

The  thunderbolt  is  often  accompanied  by  a  peculiar  sulphurous 
odor,  which  is  due  to  the  oxygen  of  the  air  becoming  electrified, 
forming  a  product  called  ozone. 

Considering  the  fearful  character  of  the  thunderbolt,  but  few- 
individuals  perish  from  it.  It  is  estimated  that  no  more  than  twenty 
deaths  a  year  occur  from  this  cause  throughout  the  whole  of  France, 
which  is  only  one  out  of  two  millions  of  inhabitants. 

Means   of  Safety. 

1O2.  It  is  recommended  to  those  who  are  fearful  of  the 
effects  of  lightning,  that  they  should  wear  clothing  of  silk, 
or  still  better,  that  they  should  sit  in  chairs  insulated  by 
glass  legs  or  upon  a  thick  plate  of  this  material.  They 
should  also  keep  as  far  as  possible  from  conductors,  par 
ticularly  the  metals.  When  thus  insulated,  even  if  struck, 
they  can  experience  only  a  slight  shock,  which  can  hardly 
prove  fatal. 

In  some  of  the  French  villages  it  is  customary  to  ring  bells  during  a 
storm,  with  the  idea  of  driving  away  the  cloud,  and  avoiding  the 
hail  which  so  frequently  accompanies  thunder  storms.  This  does  no 
good,  but  simply  exposes  the  bell  ringer  to  additional  danger,  for 
high  edifices,  like  church  spires,  are  by  far  the  most  likely  to  be 
struck,  and  as  the  bell  ropes  are  conductors  of  electricity,  the  danger 
to  those  who  hold  them  is  much  increased. 


Example.     What  is  the  cause  of  the  peculiar  odor  that  accompanies  lightning  1 
(  402.)  What  are  some  of  the  methods  of  protection  from  lightning  ? 


426 


POPULAR     PHYSICS. 


The  Return  Shock. 

4O3.  The  RETURN  SHOCK  is  a  violent,  and  often  fatal 
shock,  felt  by  men  and  animals  at  a  great  distance  from  the 
place  where  the  lightning  strikes.  (See  Fig.  280.) 

This  phenomenon  is  due  to  the  inductive  influence  exerted  by  c.i 
electrified  cloud  upon  bodies  beneath  it,  which  are  all  strongly- 
charged  with  electricity  contrary  to  that  of  the  cloud.  Now  if 
a  discharge  takes  place  at  any  point,  the  cloud  returns  to  its 
neutral  state,  induction  ceases  instantly,  and  all  of  the  bodies  elec 
trified  by  induction  instantly  return  to  a  neutral  state.  The  sud 
denness  of  this  return  is  what  constitutes  the  return  shock. 


Fig.  280. 

The  return  shock  may  be  illustrated  on  a  small  scale,  by  placing 
a  Jiving  frog  near  an  electrical  machine  in  motion.  Every  time  that 
the  machine  is  discharged  by  placing  the  finger  upon  it,  the  frog 
experiences  a  shock,  which  is  nothing  else  than  the  return  shock 
above  described. 


(  403.)  What  is  the  Kcturn  Shock  ?    Explain  its  cause.    How  illustrated  ? 


ATMOSPHERIC    ELECTRICITY. 


427 


Lightning-rods. 

4O4.  A  LIGHTNING-ROD  is  a  rod  of  metal,  placed  upon 
a  building  or  ship,  to  preserve  it  from  the  effect  of  lightning, 
as  shown  in  Fig.  281. 


Fig.  281. 

A  lightning-rod  should  fulfill  the  following  conditions : 

1.  It  should  be  of  sufficient  size. 

A  copper  rod  of  a  half  inch  in  diameter,  or  an  iron  one  of  three 
fourths  of  an  inch  in  diameter,  is  large  enough  to  protect  ai  y 
building. 

2.  If  made  of  more  than  one  piece,  the  parts  should  be 
screwed  or  welded  together,  to  avoid  defective  joints. 

(4O4.)  What  is  a  Lightning-rod  ?  What  is  the  first  condition  that  a  lightning, 
rod  should  fulfill  ?  Elustrate.  Second  condition  ? 


428  POPULAR     PHYSICS. 

3.  It  should  terminate  above  in  a  single  platinum  point. 
The  point  should  be  of  platinum  that  it  may  not  be  fused. 
It  also  prevents  the  point  from  rusting. 

4.  The  rod  should  be  carried  down  into  the  earth  till  it 
meets  with  a  good  conducting  medium,  such  as  a  layer  of 
wet  or  moist  earth. 

When  no  such  medium  can  be  reached,  a  pit  should  be  dug.  and 
after  the  lower  end  of  the  rod  has  been  carried  to  the  bottom,  it 
should  be  nearly  filled  with  some  good  conductor,  as  coke. 

The  lightning-rod  was  invented  by  FRANKLIN,  who  thought  that 
its  protective  action  consisted  in  drawing  off  the  electricity  from  the 
cloud,  and  conducting  it  to  the  earth.  The  real  explanation  of  its 
utility  is  just  the  reverse.  The  cloud  acts  by  induction  upon  the 
earth,  repelling  the  electricity  of  the  same  name  as  that  in  the  cloud> 
and  attracting  that  of  an  opposite  name,  which  accumulates  upon  the 
bodies  under  the  cloud.  Now,  by  arming  a  body  with  metallic  points 
communicating  with  the  earth,  we  permit  a  flow  of  electricity  from 
the  earth  to  the  cloud.  This  flow  not  only  prevents  the  accumula 
tion  of  electricity  upon  the  body,  but  it  tends  gradually  to  neutralize 
the  electricity  of  the  cloud  itself;  and  thus  the  rod  acts  in  a  double 
way  to  prevent  the  body  from  being  struck. 

Electrical  Meteors. 

405.  A  METEOR  is  any  atmospheric  phenomenon  ;  thus, 
wind,  rain,  snow,  hail,  thunder,  and  lightning  are  meteors. 
Besides  thunder  and  lightning,  three  other  meteors  are  at 
tributable   either  wholly  or  in   part   to  electricity;    these 
are :  hail,  tornados,  and  the  aurora  borealis. 

HaiL 

406.  HAIL  consists  of  globules  of  ice  which  fall  from  the 
clouds.     The  globules  consist  of  a  coating  of  ice,  disposed 

Third  condition  ?  Fourth  condition  ?  Who  invented  the  lightning-rod  ?  Explain 
its  mode  of  action.  (405.)  What  is  a  Meteor?  Mention  some  of  them.  (406.) 
What  is  Hail  ? 


ATMOSPHERIC    ELECTRICITY.  429 

about  a  central  nucleus  of  compact  snow.  They  are  called 
hailstones.  Hailstones  sometimes  are  very  large,  being  not 
infrequently  as  large  as  a  pigeon's  egg,  and  it  is  said  they 
sometimes  weigh  several  ounces. 

A  fall  of  hail  is  often  preceded  by  a  noise  like  that  of  rattling 
nuts  in  a  bag.  This  noise  is  attributed  to  collisions  between  the 
hailstones.  A  hailstorm  is  always  accompanied  by  electrical  phe 
nomena,  and  thunder  generally  precedes  or  accompanies  the  fall  of 
hail.  From  this  circumstance  it  is  inferred  that  hailstorms  are  in 
some  way  due  to  electrical  action.  As  yet  no  satisfactory  theory  has 
been  advanced  to  account  for  the  formation  of  hailstones,  and  espe 
cially  those  enormous  ones  that  are  sometimes  seen. 

VOLTA  supposed  them  to  be  formed  between  two  clouds  oppositely 
electrified,  and  that  they  were  alternately  repelled  from  one  to  the 
other,  like  electrical  puppets,  during  which  time  they  were  continu 
ally  increasing  in  size  by  congealing  the  moisture  of  the  clouds  upon 
their  surface,  till  at  last  they  became  heavy  enough  to  break  through 
the  lower  cloud  and  descend  to  the  earth.  This  theory  is  now 
rejected. 

The  Tornado. 

4O7.  A  TORNADO  is  a  violent  whirlwind,  attended  with 
rain,  thunder,  and  lightning.  Tornados  often  travel  con 
siderable  distances,  overturning  buildings  and  uprooting 
trees ;  they  are  accompanied  with  a  noise  like  that  of 
heavily-loaded  carts  driven  over  a  stony  road.  The  flashes 
of  lightning  and  balls  of  electrical  fire  that  accompany 
tornados,  indicate  their  electrical  origin. 

Two  species  of  tornado  are  recognized :  terrestrial  and  marine. 
according  as  they  take  place  on  land  or  on  water.  The  latter  class 
present  remarkable  phenomena.  The  rotary  force  of  the  wind  raises 
the  water  in  the  form  of  a  cone,  whilst  a  second  cone  forms  in  the 
cloud,  having  its  apex  downwards.  These  cones  move  to  meet  each 
other,  forming  a  column  of  water  reaching  from  the  ocean  to  the 

Describe  a  hailstone.  Explain  the  rattling  sound  preceding  a  hailstorm.  What 
wan  VOLTA'S  theory  of  the  formation  of  hail  ?  ( 407.)  What  is  a  Tornado  ?  Why 
is  it  regarded  as  of  electrical  origin  ?  How  many  species  of  tornados  ? 


430 


POPULAR     PHYSICS. 


cloud.     In  this  form  the  column  of  fluid  is  called  a  water-spout 
When  a  water-spout  strikes  a  ship,  it  does  immense  damage. 

The   Aurora  Borealis. 

4O8.  The  AURORA  is  a  luminous  phenomenon,  which 
appears  most  frequently  about  the  poles  of  the  earth,  and 
more  particularly  about  the  boreal  or  northern  pole,  whence 
its  name. 

At  the  close  of  twilight,  a  vague  and  dim  light  appears  in  the 
horizon  in  the  direction  of  the  magnetic  meridian.  This  light  gradu 
ally  assumes  the  form  of  an  arch  of  a  pale  yellowish  color,  having 
its  concave  side  turned  towards  the  earth.  From  this  arch  streams 
of  light  shoot  forth,  passing  from  yellow  to  pale  green,  and  then  to 
the  most  brilliant  violet  purple.  These  rays  or  streams  of  light 
generally  converge  to  that  point  of  the  heavens  which  is  indicated 
by  the  dipping  needle,  and  they  then  appear  1o  form  a  fragment  of 
an  immense  cupola,  as  shown  in  Fig.  28^. 


Fig.  282. 


TVhat  is  a  water-spout?     (4O8.)  What  is  the  Aurora?    Describe  it. 


ATMOSPHERIC    ELECTRICITY.  4:31 

Since  the  aurora  is  always  accompanied  by  a  disturbance  of  the 
magnetic  needle,  and  is  generally  arranged  in  the  direction  of  the 
dip,  and  because  the  chemical  action  of  electricity  is  accompanied 
by  precisely  analogous  phenomena,  it  is  inferred  that  it  is  due  to 
electrical  action.  Such  is  at  present  the  generally  received  belief. 


Why  is  it  regarded  as  of  electrical  origin  1 


CHAPTER  IX. 

DYNAMICAL          ELECTRICITY. 
I. — FUNDAMENTAL      PRINCIPLES. 

Galvani's  Experiment. 

4O9.  IT  lias  been  observed  that  chemical  combinations 
are  sources  of  electricity.  The  form  of  electricity  thus 
developed  is  different,  but  its  nature  is  the  same  as  that 
produced  by  friction.  The  name  of  GALVANISM  has  been 
given  to  electricity  developed  by  certain  chemical  combina 
tions,  in  honor  of  GALVANI,  who  first  discovered  this  new 
way  of  generating  it. 

In  1790,  GALVANI  observed  that  the  body  of  a  frog  recently  killed, 
when  placed  near  an  electrical  machine,  manifested  signs  of  excita 
tion  whenever  sparks  were  drawn  from  it.  The  cause  of  action  was, 
in  fact,  the  return  shock,  as  has  been  explained  ;  but  GALVANI, 
ignorant  of  this  fact,  began  to  seek  for  an  explanation  of  the  phe 
nomena.  One  day  he  saw  a  dead  frog  suspended  from  a  copper 
hook  in  a  window,  and  noticed  a  muscular  contraction  whenever  the 
wind  blew  the  lower  extremities  against  the  iron  bars  of  the  window. 
Here  was  a  case  of  electrical  manifestation  which  was  entirely 
independent  of  any  electrical  machine,  and  it  furnished  a  clew  to  one 
of  the  most  important  discoveries  in  modern  science. 

This  discovery  led  to  an  experiment  which  may  be  repeated  as 
follows :  Having  killed  a  frog  and  cut  off  the  hinder  half  of  the 
body,  we  suspend  it  by  a  copper  hook,  c,  passed  between  the  back 

(  409.)  What  is  Galvanism  ?  Why  so  called  ?  Explain  the  method  of  its  dis 
covery.  How  may  GALVANI'S  experiment  ~be  repeated  ? 


GENERAL    PRINCIPLES     OF    GALVANISM. 


433 


bone  and  the  nerves  which  run  on  each  side  of  it,  as  shown  in 
Fig.  283  ;  then  holding 
a  small  plate  of  zinc, 
z.  in  the  hand,  we 
bring  one  end  of  it  in 
contact  with  the  copper 
stem  that  holds  the 
hook,  and  then  touch 
the  legs  of  the  frog  with 
the  other  end.  At  every 
contact  the  muscles  con 
tract,  reproducing  all 
the  motions  of  life. 

GA.LVANI  attributed  the 
phenomena  observed,  to 
the  electricity  existing  in 
animal  tissues,  which, 
passing  from  the  nerves 
to  the  muscles,  through 
the  metals,  produced  the 
muscular  contractions. 


Volta's  Theory  of  Contact. 

41O.  VOLTA  repeated  the  experiment  of  GALVANI,  and 
after  much  study,  advanced  the  theory  of  contact.  Accord 
ing  to  this  theory,  when  two  metals  or  other  dissimilar  sub 
stances  are  simply  brought  in  contact,  there  is  always  a 
decomposition  of  the  natural  electricity  of  both  bodies,  the 
positive  fluid  going  to  one  and  the  negative  fluid  to  the 
other. 

In  the  case  of  the  frog,  the  electricity  was  supposed  to  be  de 
veloped  by  the  contact  of  the  copper  hook  and  zinc  plate,  the  nerves 
and  muscles  serving  simply  as  conductors. 

VOLTA  called  the  force  which  separates  the  two  electricities  in 
cases  of  contact,  the  electro-motive  force,  which  he  supposed  to  act 

To  what  diet,  GALYANI  attribute  the  phenomena  observed?  (410.)  What  w&a 
YOVTA'S  theory?  What  is  the  electro-motive  force  ? 

19 


4:34  POPULAK     PHYSICS. 

like  the  coercive  force  in  magnetism,  to  prevent  a  recombination  of 
the  separated  fluids.  He  called  those  bodies  which  by  contact 
developed  much  electricity,  good  electro-motors,  and  those  which 
developed  but  little,  he  called  bad  electro-motors.  The  best  electro 
motors  are  zinc  and  copper  soldered  together. 

In  confirmation  of  his  theory,  VOLTA  performed  the  experiment 
explained  in  speaking  of  the  condensing  electrometer,  Figs.  273  and 
274.  This  decisive  experiment  overthrew  the  theory  of  GALVANI. 
The  theory  of  contact  has  since  given  way  to  the  chemical  theory, 
which  will  be  explained  hereafter. 


The  Voltaic  File. 

411.  In  the  year  1800,  VOLTA  invented  an  apparatus 
by  which  he  could  multiply  the  number  of  contacts,  and  thus 
produce  a  more  powerful  effect.  This  apparatus  is  called 
the  voltaic  pile. 

The  voltaic  pile  has  received  many  different  forms,  but  the  same 
principle  is  applied  in  all.  One  of  these  is  shown  in  Fig.  284.  It 
consists  of  an  assemblage  of  couples,  each  consisting  of  a  disk  of  cop 
per  and  a  disk  of  zinc  in  contact,  and  each  couple  being  separated 
from  the  next  by  a  layer  of  cloth  moistened  with  dilute  sulphuric 
acid.  The  couples  are  all  disposed  in  the  same  order,  the  zinc  of 
each  couple  being  always  on  the  same  side  of  the  corresponding  disk 
of  copper.  When  the  pile  is  completed,  there  will  bt  a  disk  of  zinc 
at  one  end  and  a  disk  of  copper  at  the  other.  A  connection  is  made 
between  them  by  means  of  the  wires,  a  and  b,  one  being  attached  to 
each  of  the  extreme  plates. 

In  the  pile  shown  in  Fig.  284,  there  are  twenty  couples,  the  zinc 
disk  being  at  the  bottom  of  each  couple,  and  the  copper  one  at  the 
top.  The  pile  is  supported  by  a  suitable  frame-work. 

This  apparatus  has  been  much  modified,  but  the  name  pile  has 
been  retained  for  all  apparatus  of  the  same  kind,  and  the  electricity 
generated  in  this  way  is  called  voltaic,  or  galvanic  electricity. 


What  arc,  good  and  bad  electro-motors?  (411.)  What  is  the  voltaic  pile? 
Describe  the  pile  figured  in  the  text.  What  name  is  given  to  the  electricity  0} 
the,  pile  ? 


GENERAL    PRINCIPLES     OF     GALVANISM. 


435 


Fig.  284. 

Electrical  Tension  in  the  File.  —  Poles.  —  Electrodes. 

412.  In  a  pile  which  is  insulated,  one  half  is  found  to  be 
electrified  positively  and  the  other  half  negatively,  the  middle  being 
neutral.  In  the  zinc  and  copper  pile,  that  end  towards  which  the  zinc 
plate  in  each  couple  is  turned,  is  positive,  the  other  end  being  nega 
tive,  as  indicated  by  the  signs  +  and  — ,  in  Fig.  284. 

The  tension  of  the  electricity  in  either  end  increases  with  the 
number  of  couples  in  the  pile,  but  is  independent  of  their  size.  The 
tension  is  greatest  at  the  two  extremities ;  hence  these  extremities 
are  named  poles ;  the  one  towards  the  zinc  end  is  the  positive  pole, 
the  one  towards  the  copper  end  is  the  negative  pole. 

The  wires,  a  and  6  (Fig.  284),  which  are  attached  at  the  two 
poles  for  the  purpose  of  completing  the  circuit,  are  called  electrodes. 


(412.)  How  does  the  tension  vary  in  the  pile?     Where  is  it  greatest  in  any 
pile  ?     What  are  the  poles  ?    ffow  named  ?     What  are  the  electrodes  ? 


4:36  POPULAR     PHYSICS. 


Electrical  Currents. 

413.  So  long  as  the  electrodes  remain  separated,  the 
pile  manifests  no  electrical  action,  but  on  being  brought 
near  each  other,  a  small  spark  is  seen  to  pass,  which  arises 
from  a  recombination  of  the  two  electricities.  The  passage 
of  the  spark  does  not  discharge  the  pile,  as  is  the  case  with 
the  Ley  den  jar.  We  see  a  continual  succession  of  sparks, 
showing  that  the  process  of  decomposition  is  continually  kept 
up  in  the  pile,  by  which  the  poles  are  continually  fed  with 
new  supplies  of  the  positive  and  negative  fluids. 

If  the  two  wires  are  brought  into  actual  contact,  the 
sparks  cease,  but  the  flow  of  the  fluids  continues  as  before, 
decomposition  going  on  in  the  pile,  and  recomposition  taking 
place  through  the  electrodes.  This  continuous  flow  of 
electricity  is  called  the  electric  current.  There  are,  in  fact, 
two  currents  flowing  in  opposite  directions,  according  to  the 
two  fluid  theory,  but  it  is  found  convenient  to  consider  only 
one  of  them,  namely,  that  which  flows  from  the  positive  to 
the  negative  pole.  In  the  figures,  hereafter,  the  direction 
of  this  "current  will  be  indicated  by  an  arrow,  as  in  Fig.  292. 

Chemical  Theory   of  the  Pile. 

414  FABKONI  first  suggested  that  the  phenomena  of 
the  pile  were  due  to  chemical  action.  In  the  pile  described, 
the  dilute  acid  in  the  cloths  between  the  couples,  acting  upon 
the  zinc,  was  supposed  to  be  the  cause  of  the  development 
of  electricity.  This  view  was  adopted  by  DAVY  and  WOL- 
LASTON,  who  made  many  experiments  calculated  to  sustain 
it.  Finally,  DE  LA  RIVE  and  BECQUEHEL  succeeded^  11 
demonstrating  most  conclusively  that  in  every  chemical 
action  electricity  is  developed.  They  also  showed  that 

(413.1  What  phenomenon  is  observed  when  the  circuit  is  completed?    What ;  is 
the  electric  current  ?    Which  way  do  we  suppose  the  current  to  flow  ?    (  41 4.)  v 
was  FABRONI'S  theory  ? 


GENERAL     PRINCIPLES     OF     GALVANISM. 


437 


whenever  a  metal  is  attacked  by  an  acid,  the  former  is 
positively  and  the  latter  negatively  electrified. 

According  to  this  view,  which  is  now  universally  adopted, 
the  acidulated  cloths,  that  YOLTA  regarded  as  simply  con 
ductors,  are  in  fact  the  principal  cause  of  the  development 
of  electricity. 

The  Carbon  Pile. 

415.  The  CAKBON  PILE  was  invented  by  BUNSEST,  about 
twenty  years  ago,  and  is  often  called  the  Jlunsen  Pile. 

Each  couple  of  BUXSEN'S  pile  consists  of  four  pieces,  which 
are  shown  both  separately  and  united  in  Fig.  285.  These 


Fig.  285. 

parts  are:  1.  An  earthen  vessel,  A,  containing  dilute  sul 
phuric  acid ;  2.  a  zinc  cylinder,  .Z?,  open  at  one  side  and 
having  a  strip  of  copper  soldered  to  its  upper  extremity ; 
3.  a  vessel,  (7,  of  porous  earthen-ware,  containing  nitric 
acid  ;  4.  a  cylinder  of  carbon  or  coke,  which  is  well  calcined, 
and  a  good  conductor  of  electricity.  At  the  top  of  this 

How  is  the  action  of  the  pile  explained  according  to  this  theory?  (415-VWho 
invented  the  Carbon  Pile?  When?  Describe  one  of  the  couples  of  BUNSEN'S  pile, 
In  detail. 


438  POPULAR     PHYSICS. 

cylinder  a  stem  of  copper  is  inserted,  to  which  is  soldered  a 
thin  strip  of  the  same  metal. 

The  completed  couple,  represented  at  P,  is  formed  by 
putting  the  cylinder  J3  into  A,  then  putting  C  into  _Z>,  and 
finally,  introducing  the  cylinder  D  into  the  cylinder  C.  On 
bringing  the  slips  of  copper  in  contact,  a  current  of  elec 
tricity  is  developed,  flowing  from  the  carbon  to  the  zinc. 

In  this  case  there  is  a  double  chemical  action.  Water  is  decom 
posed  in  the  vessel  A,  giving  its  oxygen  to  the  zinc,  forming  oxyde  of 
zinc,  which  is  taken  up  by  the  sulphuric  acid,  producing  sulphate  of 
zinc.  This  remains  in  solution.  The  hydrogen  of  the  water  passes 
through  the  porous  cell,  C,  and  uniting  with  a  part  of  the  oxygen  of 
the  nitric  acid,  decomposes  it.  reproducing  water,  and  also  forming 
nitrous  acid,  which  escapes  in  fumes.  This  double  action  develops 
a  large  amount  of  electricity,  that  flows  from  the  carbon,  which  is 
the  positive,  towards  the  zinc,  which  is  the  negative  pole  of  the  couple. 

Any  number  of  couples  may  be  united  by  attaching  the 
copper  slip  of  the  zinc  cylinder  in  one  couple  to  that  of  the 
carbon  in  the  next  couple,  and  so  on  throughout  the  com 
bination.  The  remaining  two  slips,  which  will  be  at  the 
extreme  ends  of  the  combination,  may  be  united  by  a  con 
ductor. 

Such  a  combination  is  called  a  galvanic  battery,  or  some 
times  a  voltaic  battery.  A  galvanic  battery  has  been  con 
structed,  containing  as  many  as  eight  hundred  couples. 
Fig,  286  represents  a  battery  of  twenty  couples. 


II.  —  APPLICATIONS      OF      GALVANIC      ELECTRICITY. 

Effects   of  the  Galvanic  Battery. 

416.     The  EFFECTS  OF  THE  GALVANIC  BATTERY  may,  for 
convenience  of  study,  be  divided  into  physiological,  heat- 
Explain  the  action  of  a  couple  of  BITNBEN'S  pile.    How  are  the  couples  con 
nected  ?   What  is  such  a  combination  called  ?    (4 1 6.)  What  are  the  principal  effects 
of  the  galvanic  battery  ? 


APPLICATIONS     OF    GALVANIC    ELECTRICITY.  439 

ing,  illuminating,  chemical,  and  magnetic.  They  are  all 
due  to  the  recombination  of  the  two  electricities,  as  in 
machine  electricity,  but  they  are  more  remarkable  and  more 
energetic,  because  of  their  continuous  action. 

Physiological  Effects. 

417.  The  PHYSIOLOGICAL  EFFECTS  of  galvanic  electricity 
are  a  succession  of  shocks  producing  violent  muscular  con 
tractions,  not  only  in  living,  but  in  dead  animals,  as  shown 
in  the  case  of  GALVANI'S  frog. 

When  we  touch  but  one  of  the  poles  of  a  galvanic  battery,  no 
shock  is  felt,  but  if  we  take  both  electrodes  in  our  hands,  as  in  Fig. 
286,  we  feel  a  sensation  similar,  to  a  shock  from  a  Lcyden  jar.  with 


Fig.  286. 


To  what  are  they  due  ?    (417.)  What  are  the  physiological  effects  ?    How  may  a 
shock  be  obtained  from  a  battery  ? 


4:4:0  POPULAR     PHYSICS. 

this  difference,  the  latter  is  instantaneous,  whilst  that  from  the  gal 
vanic  battery  is  continuous.  The  action  of  the  battery  keeps  up  a  con 
tinuous  supply  of  the  two  fluids,  which  supplies  the  place  of  that  lost 
by  recombination  in  passing  through  the  body  of  the  experimenter. 
The  effect  of  galvanic  electricity  upon  the  bodies  of  dead  animals 
is  peculiarly  striking.  It  produces  violent  contractions  of  the 
muscles,  causing  motions  similar  to  those  of  the  living  being.  On 
the  occasion  of  performing  some  experiments  upon  the  body  of  a 
criminal  who  had  been  executed,  in  England,  a  violent  and  convul 
sive  respiration  was  produced,  the  eyes  opened,  the  lips  moved,  and 
the  face,  no  longer  under  the  control  of  the  will,  assumed  expressions 
so  strange  and  horrifying  that  one  of  the  assistants  fainted  from  ter 
ror,  and  only  recovered  his  natural  state  of  mind  after  several  days. 


Heating  Effects. 

418.  "When  a  current  of  galvanic  electricity  is  passed 
through  a  conductor,  it  becomes  heated,  and  often  to  such 
a  degree  as  to  produce  fusion  or  even  vaporization.  When 
a  powerful  current  is  passed  through  a  wire,  it  soon  becomes 
incandescent,  and  then  melts  or  is  dispersed  in  vapor. 
Small  wires  burn  with  splendid  brilliancy.  Silver  burns 
with  a  greenish  light,  and  much  smoke  arising  from  the 
vaporization  of  the  metal.  Gold  burns  with  a  bluish  white 
light.  Platinum,  which  is  infusible  in  the  most  intense  heat 
of  our  furnaces,  melts  into  spherical  globules  with  a  dazzling 
light. 

With  a  battery  of  600  couples,  DESPKETZ  fused  nearly 
half  a  pound  of  platinum  in  a  few  minutes.  Carbon  is  the 
only  body  which  has  not  been  fused  by  galvanic  electricity. 
DESPRETZ,  however,  by  passing  a  current  through  small 
rods  of  pure  carbon,  succeeded  in  softening  them  so  much 
that  they  could  be  bent  and  made  to  adhere,  which  indicates 
an  approach  to  fusion. 


What  e feet  has  galvanic  electricity  on  dead  animals  ?    '418.)  Describe  the 
heating  effects  of  the  battery.    DESPRETZ'  experiments. 


APPLICATIONS    OF    GALVANIC    ELECTRICITY.  441 

Illuminating  Effects. 

419.  The  heating  effects  just  described  are  accompanied 
with  a  disengagement  of  more  or  less  light,  but  to  obtain 
the  most  brilliant  electrical  light  possible,  calcined  carbon 
points  are  employed.  They  are  at  first  placed  in  contact, 
one  being  connected  with  the  positive,  and  the  other  with 
the  negative  pole  of  a  powerful  galvanic  battery,  as  shown 
in  Fig.  287.  The  points  immediately  become  incandescent, 


(419.)  Describe  the  illuminating  effects. 
19* 


442  POPULAR    PHYSICS. 

emitting  a  light  of  dazzling  brightness.  If  the  points  are 
slightly  separated,  the  current"  still  continues  to  pass  be 
tween  them,  and  the  light  takes  the  form  of  a  luminous 
arch,  called  the  voltaic  arch.  In  this  experiment  the  point 
connected  with  the  positive  pole  wastes  away,  whilst  the 
other  increases  in  size ;  hence  we  conclude,  that  particles  of 
carbon  are  transported  from  the  former  to  the  latter ;  this 
explains  how  the  current  continues  to  pass  in  spite  of  the 
interval  which  separates  them. 

The  intensity  of  the  electrical  light  is  very  great.  A  battery  of 
48  small  couples  furnishes  a  light  equal  to  that  of  572  wax  candles ; 
with  100  couples  a  light  is  produced  so  intense  as  to  dazzle  the  eyes, 
and  with  600  couples  the  intensity  is  such  as  to  render  it  as  impos 
sible  to  look  at  it,  as  it  is  to  look  at  the  sun. 

In  1844.  FOUCAULT  first  made  use  of  the  electrical  light  instead  of 
that  of  the  sun,  to  illuminate  the  solar  microscope.  Since  then, 
many  attempts  have  been  made,  and  with  some  success,  to  apply  it 
to  purposes  of  general  illumination.  Fig.  287  represents  an  appa 
ratus  employed  for  the  purpose  of  illumination.  The  battery  is  con 
tained  in  the  interior  of  a  cast-iron  pedestal,  upon  which  is  erected 
a  column  of  the  same  material.  At  the  top  of  the  column  are  two 
carbon  points,  one  connected  with  each  pole  of  the  battery  by  copper 
wires,  insulated  by  gutta  percha  coverings. 

Chemical  Effects. 

I2O.  The  most  important  chemical  effects  produced  by 
galvanic  electricity,  are  the  decomposition  of  bodies  traversed 
by  it,  and  the  transportation  of  their  elements. 

In  order  to  understand  the  chemical  effects,  we  must  explain  the 
meaning  of  certain  terms  employed  in  chemistry. 

Oxydcs  are  generally  compounds  of  oxygen  with  the  metalsT  Thus, 
iron  rust  is  an  oxyde  of  iron,  that  is,  it  is  a  compound  of  oxygen  and 
iron  :  vermilion  is  an  oxyde  of  lead  :  potash  is  an  oxyde  of  potassium, 
and  so  on. 

What  is  the  voltaic  arch?  Illustrate  by  example.  Describe  FOTTCATTLT'S  experi 
ment.  Describe  the  apparatus  for  illumination.  ( -120.)  What  are  the  most  im 
portant  chemical  effects?  What  are  ovydes  f 


APPLICATIONS     OF     GALVANIC    ELECTKICITY. 


443 


Acids  are  generally  compounds  of  oxygen  with  some  non-metallic 
body.  Thus,  sulphuric  acid  is  a|pompound  of  oxygen  and  sulphur; 
nitric  acid  is  a  compound  of  oxygen  and  nitrogen  ;  carbonic  acid  is 
a  compound  of  oxygen  and  carbon,  and  so  on. 

Salts  are  generally  compounds  of  an  acid  with  an  oxyde.  Thus, 
sulphate  of  potash  is  a  compound  of  sulphuric  acid  and  potash, 
nitrate  of  copper  is  a  compound  of  nitric  acid  with  an  oxyde  of  cop 
per,  and  so  on. 

In  these  definitions,  we  say  generally,  because  the  definitions  given 
are  only  intended  for  illustration,  and  it  is  not  thought  best  to  enter 
into  a  detailed  account  of  the  various  substances  described.  That 
belongs  to  Chemistry. 

Decomposition  of  Water. 

421.  A  galvanic  current  was  first  employed  to  decom 
pose  water  in  the  year  1800,  by  CARLISLE  and  NICHOLSON. 


H    o 


Fig.  288. 

To  repeat  the  experiment,  we  may  employ  the  apparatus 
shown  in  Fi<r.  288.  It  consists  of  a  glass  dish  with  a  wooden 
bottom.  Rising  from  the  bottom  are  two  platinum  wires, 

Acid*?  Snit*?  (421.)  When  was  water  first  decomposed?  How  may  the  ex 
periment  Ife  repeated  ? 


4:4:4:  POPULAR     PHYSICS. 

which  pass  through  the  wooden  stand  and  terminate  in  the 
tubes,  a  and  b.  These  wires  s<?rve  as  electrodes. 

The  glass  vessel  is  partially  filled  with  water,  to  which  a 
small  quantity  of  sulphuric  acid  is  added  to  improve  its 
conducting  power.  Two  narrow  bell-glasses,  ^and  0,  are 
filled  with  water  and  inverted  over  the  two  platinum  wires. 
The  tube,  a,  is  then  connected  with  the  positive  pole  of  the 
battery,  and  the  tube,  5,  with  the  negative  pole.  A  current 
is  set  up  from  one  wire  to  the  other  through  the  water,  and 
decomposition  begins,  as  is  shown  by  bubbles  of  gas  rising  in 
the  two  bell-glasses. 

By  testing  the  gases  thus  obtained,  we  find  that  in  the 
glass,  0,  corresponding  to  the  positive  pole  of  the  battery, 
is  pure  oxygen,  whilst  that  in  the  glass,  IT,  corresponding  to 
the  negative  pole,  is  pure  hydrogen.  We  see  also  that  the 
volume  of  hydrogen  is  twice  that  of  the  oxygen.  This 
experiment  shows  that  water  is  composed  of  oxygen  and 
hydrogen,  mixed  in  the  proportion  of  one  volume  of  the 
former  to  two  of  the  latter. 

Decomposition  of  Oxydes  and  Salts. 

422.  Oxydes  and  salts  may  in  like  manner  be  decom 
posed  by  a  current  of  electricity.  In  decomposing  oxydes, 
the  oxygen  is  transported  to  the  positive  electrode,  and  the 
metal  to  the  negative  electrode.  In  the  decomposition  of 
acids  there  is  a  like  transfer  of  elements,  the  oxygen  going 
to  the  positive,  and  the  other  component  to  the  negative 
electrode. 

In  decomposing  salts,  there  maybe  several  cases.  Some 
times  there  is  a  simple  resolution  of  the  salt  into  an  acid  and 
an  oxyde,  in  which  case  the  acid  goes  to  the  positive  and  the 
oxyde  to  the  negative  electrode.  Sometimes,  besides  the 
separation  into  acid  and  oxyde,  the  latter  is  decomposed ; 

Explain  in  detail.    (422.)  How  may  oxydes  and  salts  be  decomposed?    Explain 
the  different  cases  of  decomposition  of  salts. 


APPLICATIONS     OF     GALVANIC     ELECTRICITY.  445 

in  this  case  the  oxygen  and  the  free  acid  go  to  the  positive, 
and  the  metal  alone  goes  to  the  negative  electrode.  This 
action  is  utilized  in  the  process  of  elecjtrotyping. 

DAVY,  at  ihe  beginning  of  the  present  century,  used  the 
battery  to  decompose  potash,  soda,  lime,  baryta,  magnesia, 
alumina,  &c.,  and  thus  demonstrated  that  all  of  these  sub 
stances,  previously  regarded  as  simple  bodies,  were  in 
reality  compound.  They  consist  of  oxygen  united  with 
metals,  which  are  called  respectively,  potassium,  sodium, 
calcium,  barium,  magnesium,  aluminium,  <fcc. 

Application  of  Electricity  to  Galvanoplasty. 

423.  GALVANOPLASTY,  or  ELECTROTYPING,  is  the  opera 
tion  of  copying  medals,  statues,  and  the  like,  in  metal,  by 
the  aid  of  galvanic  electricity. 

Such  copies  were  formerly  made  by  the  process  of  casting ; 
now  they  are  in  many  cases  more  elegantly  produced  by 
galvanoplasty.  This  process  was  discovered  simultaneously 
by  SPENCER,  of  London,  and  JACOBI,  of  St.  Petersburg,  in 
1838,  the  year  preceding  the  discovery  of  the  daguerreotype 
process. 

Method  of  Electrotyping. 

424.  The  first  step  is  the  preparation  of  a  mould  of  the 
object,  upon  the  accuracy  of  which  depends  the  success  of 
the  entire  operation.     Wax,  plaster,  or  gutta  percha  may 
be  used,  but  the  latter  material  is  now  considered  the  best. 
At  ordinary  temperatures,  gutta  percha  is   hard,  but  on 
being  heated,  it  becomes  soft  and  ductile.     To  form  the 
mould,  the  gutta  percha  is  warmed  by  putting  it  into  a 
vessel  of  warm  water  and  allowing  it  to  remain  till  it  is  of 
the  proper  softness  ;  it  is  then  placed  upon  the  object  to  be 

Explain  DAVY'S  experiments.  (423.)  What  is  Galvanoplasty  ?  When  discovered  ? 
By  whom?  (424.)  Explain  the  operation  of  electro  typing,  in  detail. 


446 


POPULAR    PHYSICS. 


copied,  and  pressed  with  the  fingers  till  it  touches  every 
point  of  the  surface  of  the  object,  when  it  is  left  to  harden 
by  cooling.  After  hardening,  it  is  removed  and  is  ready  for 
use.  As  gutta  percha  is  apt  to  adhere  to  certain  bodies, 
precaution  should  be  taken  to  cover  them  with  a  thin  layer 

of  powdered  plumbago  or 
graphite.  This  may  be 
laid  on  with  a  soft  brush, 
and  if  properly  applied,  it 
effectually  prevents  the  ad 
hesion  of  the  mould. 

The  second  step  is  to 
deposit  the  metal  in  the 
mould.  As  gutta  percha 


f 


Fig.  289. 


Fig.  290. 


is  a  non-conductor  of  electricity,  it  is  necessary  to  cover  it 
with  a  conducting  substance.  This  is  done  by  laying  on  a 
coating  of  plumbago  in  the  same  manner  as  in  forming  the 
mould.  The  mould  thus  prepared  is  then  made  ready  for 
the  bath  by  attaching  to  it  the  proper  suspending  wires,  as 
shown  in  Fig.  289. 

Fig.  290  represents  one  face  of  a  medal  to  be  copied, 
and  Fig.  289  represents  the  gutta  percha  mould  prepared 
for  receiving  the  metallic  deposit,  For  making  the  deposit, 


APPLICATIONS     OF     GALVANIC    ELECTRICITY. 


447 


which  we  shall  suppose  to  be  of  copper,  a  DANIEL'S  battery 
of  two  or  three  couples  is  usually  employed. 

A  couple  of  DANIEL'S  battery  differs  from  one  of  BUNSEN'S  in  the 
following  particulars.  The  carbon  cylinder  is  replaced  by  one  of 
zinc,  denoted  by  Z,  in  Fig.  291,  and  the  zinc  cylinder  is  replaced  by 
one  of  copper,  denoted  by  C,  in  the  same  figure.  The  outer  vessel 
is  of  glass,  and  is  filled  with  a  solution  of  sulphate  of  copper  (blue 
vitriol),  which  is  kept  saturated  by  some  crystals  of  the  sulphate 


Fig.  291. 

placed  at  the  bottom  of  the  vessel.  The  porous  vessel  is  filled  with 
dilute  sulphuric  acid.  When  this  battery  is  in  action,  water  is 
decomposed :  the  oxygen  goes  to  the  zinc,  forming  oxyde  of  zinc, 
which  is  dissolved  by  the  sulphuric  acid,  giving  sulphate  of  zinc. 
The  hydrogen  of  the  water  goes  to  the  sulphate  of  copper  in  P.  and 
decomposes  it.  The  result  of  these  decompositions  and  recornposi- 
tions  is  to  keep  up  a  current  of  electricity,  as  shown  by  the  arrows, 
which  will  continue  as  long  as  the  vessel.  P,  is  kept  full  of  the 
saturated  solution  of  sulphate  of  zinc. 

Fig.  291  shows  the  method  of  depositing  the  metal  upon 

What  kind  of  a  battery  is  used  for  depositing  copper?    Explain  one  of  DANIEL'S 
coitplcs.    Explain  Fig.  291. 


448  POPULAR    PHYSICS. 

the  mould.  J/is  a  vessel  filled  with  a  solution  of  sulphate 
of  copper ;  A  and  B  are  metallic  rods  communicating  with 
the  two  poles  of  the  battery  ;  the  mould  is  suspended  from 
the  rod,  J5,  and  facing  it  is  a  plate  of  pure  copper  suspended 
from  the  rod,  A  ;  these  constitute  the  electrodes,  the  mould 
being  the  negative  one. 

The  current  which  is  set  up  through  the  solution  of  cop 
per  between  the  electrodes,  decomposes  the  sulphate  into 
sulphuric  acid,  oxygen,  and  pure  copper.  The  sulphuric 
acid  and  oxygen  go  to  the  copper  plate,  and  uniting  with  it, 
produce  sulphate  of  copper ;  the  pure  copper  goes  to  the 
negative  electrode,  that  is,  to  the  mould,  and  is  there  depos 
ited.  After  about  two  days  the  coating  of  copper  becomes 
thick  enough  to  be  removed  from  the  mould,  and  it  then 
presents  a  fac-simile  of  the  object  to  be  copied.  In  copying 
medals,  each  face  is  copied  separately,  and  the  two  are  united 
by  means  of  some  fusible  metal  placed  between  them 

Electro-gilding  and  Electro-plating. 

425.  The  process  of  covering  bodies  with  thin  coatings 
of  gold  or  silver  is  analogous  to  that  of  electrotyping.  The 
perfection  of  the  process  consists  in  making  the  coating  of 
gold  or  silver,  not  only  of  uniform  thickness,  but  also  closely 
adherent. 

The  object  to  be  gilded  or  silvered  is  first  heated  upon 
a  charcoal  fire  to  remove  all  fatty  matter  ;  it  is  next  plunged 
into  dilute  sulphuric  acid,  and  then  rubbed  with  a  hard 
brush  to  remove  any  oxyde  that  may  exist  upon  the  surface ; 
it  is  next  plunged  into  common  nitric  acid,  and  then  into 
nitric  acid  into  which  a  small  quantity  of  salt  and  soot  has 
been  thrown  ;  it  is  then  washed  in  pure  water  and  carefully 
dried  in  sawdust,  and  is  ready  for  use. 


Explain  the  chemical  chancres  which  take  place.    ( 425.)  What  is  the  process  of 
electro-silvering  and  electro-gilding?    How  is  the  object  cleaned  ? 


APPLICATIONS     OF    GALVANIC    ELECTRICITY. 


449 


The  method  of  silvering,  or  electro-plating,  is  shown  in 
Fig.  292.  The  object  to  be  silvered  is  suspended  in  a  bath 
of  a  silver  solution  by  a  metallic  rod  which  connects  with  the 
negative  pole  of  a  BUNSEN'S  battery.  Immediately  below  it 
is  a  plate  of  pure  silver,  which  is  connected  with  the  positive 
pole  of  the  battery.  The  object  to  be  silvered  and  the  silver 
plate,  «,  constitute  the  electrodes,  a  being  the  positive  one. 
The  explanation  of  the  process  is  analogous  to  that  in  the 
preceding  article. 


. 

fc'\^lr5- 


Fig.  292. 

The  salt  of  silver  generally  employed  is  a  double  cyanuret  of  sil 
ver  and  potassium.  The  thickness  of  the  coating  deposited  will 
depend  upon  the  power  of  the  battery  and  upon  the  time  of  immer 
sion. 

The  process  of  gilding  is  the  same  as  that  of  silvering, 
except  that  we  use  a  cyanuret  of  gold  and  potassium,  and  a 
plate  of  gold  at  «,  instead  of  a  silver  one. 


Explain  the  process  of  silvering  as  shown  in  Fig.  292.     What  salt  of  silver  is  em 
ployed  f    What  is  the  process  of  gilding  ?    What  salt  of  gold  is  used  ? 


450  POPULAR     PHYSICS. 

The  history  of  electro-plating  and  electro-gilding  is  briefly  as  fol 
lows  :  In  1803,  BRUGNATELLI  first  gilded  a  silver  medal  by  suspend 
ing  it  in  a  solution  of  gold  from  the  negative  pole  of  a  battery,  but 
proceeded  no  further.  In  1840,  DE  LA  RIVE,  of  Geneva,  discovered 
a  process  of  gilding  metals  with  a  battery,  but  by  his  process  much 
gold  was  wasted,  and  the  work  was  unsatisfactory.  In  the  same 
year,  ELKINGTON,  an  Englishman,  discovered  the  process  of  gilding 
by  means  of  the  cyanuret  of  gold  and  potassium.  A  few  months 
later,  RUOLZ  succeeded  in  silvering  and  platinizing  metals  by  the 
methods  now  in  general  use.  The  arts  of  electro-plating,  electro- 
gilding,  and  electrotyping  are  now  of  general  application,  and  afford 
occupation  to  thousands  of  artisans. 

Give  an  outline  of  the  history  of  electro-plating  and  electro-gilding. 


CHAPTER  X. 

ELECTRO-MAGNETISM. 

I. — FUNDAMENTAL         PRINCIPLES 

Relation  between  Magnetism  and  Electricity. 

426.  IT  was  observed  at  an  early  period  that  the  mag 
netic  and  electrical  fluids  had  many  analogous  properties. 
In  each  case  fluids  of  the  same  name  repel,  whilst  those  of 
an  opposite  name  attract.  It  was  also  observed  that  a 
stroke  of  lightning  often  reversed  the  poles  of  a  magnetic 
needle,  and  sometimes  completely  destroyed  its  magnetism. 
The  two  have  also  points  of  dissimilarity.  Magnetic  fluids 
are  not  transmitted  like  electrical  fluids  through  conductors. 
A  magnet  does  not,  like  an  electrified  body,  return  to  a 
neutral  state  when  brought  into  communication  with  the 
earth.  Magnetism  can  only  be  developed  in  a  few,  whereas 
electricity  may  be  developed  in  all  bodies. 

Between  these  analogies  and  dissimilarities  nothing  posi 
tive  could  be  affirmed  with  respect  to  the  identity  of  mag 
netism  and  electricity,  until,  in  1819,  ERSTED  made  a  dis 
covery  which  showed  that  these  physical  agents  are  most 
intimately  allied,  if  not  identical. 


(426.)  "What  early  observations  were  made  on  the  relation  of  the  phenomena  of 
electricity  and  magnetism  ?    What  dissimilarities  -were  noticed  ? 


452 


POPULAR     PHYSICS. 


Action  of  an  Electrical   Current  upon  a  Magnet. 

427.  ERSTED  discovered  the  fact  that  an  electrical  cur 
rent  has  a  directive  power  over  the  magnetic  needle,  tend 
ing  always  to  direct  it  at  right  angles  to  its  own  direction. 

This  action  may  be  shown  by  the  apparatus  represented 
in  Fig.  293.  If  a  wire  be  placed  parallel  to  and  pretty  near 
a  magnetic  needle, .  and  then  a  current  of  electricity  be 
passed  through  it,  the  needle  will  turn  around,  and  after  a 


Fig.  293. 

few  oscillations  will  come  to  rest  in  a  position  sensibly  at 
right  angles  to  the  current.  That  it  does  not  take  a  posi 
tion  absolutely  perpendicular  to  that  of  the  current,  is  be 
cause  of  the  directive  force  of  the  earth,  which  partially 
counteracts  that  of  the  current. 

The  direction  towards  which  the  austral  pole,  that  is,  the  north 
end  of  the  needle,  will  turn,  depends  upon  the  direction  of  the  cur- 

(427)  What  discovery  was  made  by  ERSTED?    Explain  the  action  of  the  elec 
trical  current  on  the  needle.     Which  way  does  the  north  end  turn  ? 


GENERAL    PRINCIPLES     OF    ELECTRO-MAGNETISM.         453 

rent.  If  that  flows  from  south  to  north,  and  above  the  needle,  the 
needle  deviates  towards  the  west  :  if  it  flows  towards  the  south,  and 
above  the  needle,  the  latter  deviates  towards  the  east.  When  the 
current  flows  below  the  needle,  the  phenomena  are  reversed. 

Ampere's  Law. 

428.  AMPERE,  to  whom  the  discovery  of  the  greater 
portion  of  electro-magnetic  phenomena  is  due,  gave  a  simple 
expression  to  the  law  which  governs  the  action  of  a  current 
upon  a  magnet.     He  supposes  an  observer  lying  down  upon 
the  wire  along  which  the  current  flows,  the  current  entering 
at  the  head  and  going  out  at  the  feet.     Then,  if  he  turn  his 
face  towards  the  needle,  the  austral  pole  will  in  all  cases  be 
deviated  towards  his  right  hand. 

Action   of  Magnets   upon  Currents,    and   of  Currents   upon 
Currents. 

429.  AMPERE  established  the  following  principles  : 

1.  Magnets  exercise  a  directive  force  upon  currents. 

To  illustrate  this,  we  bend  a  copper  wire  into  a  circular  form, 
and  then  dip  its  extremities,  which  should  bo  pointed  with  steel, 
into  cups  of  mercury,  one  above  the  other,  as  shown  in  Fig.  294. 
These  cups  communicate  with  the  two  poles  of  a  battery,  by  means 
of  which  a  current  may  be  generated,  flowing  as  indicated  by  the 
arrows.  Now  if  a  bar  magnet  be  brought  near  this  current,  the 
axis  being  in  the  plane  of  the  current,  we  shall  see  the  hoop  turn 
about  the  steel  points  in  the  cups,  and  come  to  rest,  with  its  plane 
perpendicular  to  the  axis  of  the  magnet.  This  experiment,  which  is 
due  to  AMPERE,  is  the  reverse  of  that  made  by  EKSTED. 

2.  The  earth,  which  acts  like  a  huge  magnet  upon  a  mag 
netic  needle,  acts  in  the  same  manner  upon  movable  cur- 


(428.)  Explain  AMPERF/S  law.    (429.)  What  is  AMPERE'S  first  principle ?    Sow 
illustrated  ?    His  second  principle  ? 


451 


POPULAR     PHYSICS. 


rents ;  that  is,  it  directs  them  so  that  they  are  perpendicular 
to  the  magnetic  meridian. 

This  may  be  shown  by  the  apparatus  of  Fig.  294.  If  the  com 
munication  with  the  battery  be  cut  off,  and  the  hoop  be  turned  till 
its  plane  coincides  with  the  magnetic  meridian,  it  will  remain  in  that 


Fig.  294. 

position.  If  now  a  current  be  passed  through  it,  we  see  it  turn 
slowly  around  the  pivots,  so  as  to  take  a  position  at  right  angles  to 
the  meridian.  It  will  turn  in  such  a  direction  that  the  current  in 
the  lower  part  of  the  hoop  will  flow  from  east  to  west. 

3.  Two  parallel  currents  attract  each  other  when  they 
flow  in  the  same  direction,  and  repel  each  other  when  they 
flow  in  opposite  directions. 

4.  If  a   wire   be    coiled 
into  a  double  helix,  as  re 
presented  in  Fig.  295,  and 
then  be  suspended  by  its 
steel  points  in  the  cups  of 
mercury  (Fig.  294)  it  will, 
when   a   current  is  passed 


Fig.  295. 


How  may  this  be  sTwum  ?    His  third  principle  ?    His  fourth  principle  ? 


GENERAL    PRINCIPLES     OF    ELECTRO-MAGNETISM.        455 

through  it,  arrange  itself  in  the  meridian  like  a  magnetic 
needle.  When  the  current  takes  the  direction  of  the 
arrows,  the  end,  a,  becomes  an  austral  pole,  and  is  directed 
towards  the  north. 

When  thus  suspended,  the  helix  has  all  the  properties  of  a  magnet^ 
and  is  subject  to  the  same  laws  of  attraction  and  repulsion.  A  helix 
of  the  kind  described  is  called  a  solenoid. 

Ampere's  Theory  of  Magnetism. 

43O.  From  the  facts  explained  in  the  last  article,  AM 
PERE  deduced  a  theory  of  magnetism.  He  supposes  mag 
netism  to  be  due  to  currents  of  electricity  flowing  around 
the  ultimate  molecules  of  a  magnet,  always  in  the  same 
direction.  The  currents  in  the  interior  of  the  magnet  neu 
tralize  each  other,  and  consequently  the  total  effect  of  all 
the  currents  in  a  magnet,  is  the  same  as  that  of  a  set  of 
surface  currents  flowing  around  the  magnet,  in  such  a  direc 
tion,  that  if  we  place  the  eye  at  the  south  end  of  a  magnet, 
and  look  in  the  direction  of  the  axis,  the  current  will  flow 
around  in  the  same  direction,  as  the  hands  of  a  watch. 

He  supposes  the  directive  force  of  the  earth  to  be  due  to 
currents  of  electricity  flowing  around  it,  parallel  to  the 
magnetic  equator,  from  east  to  west.  These  currents  are 
produced  by  variations  of  temperature,  which  arise  from  the 
earth's  revolution  continually  presenting  a  new  portion  to 
the  direct  action  of  the  sun's  rays. 

If  we  conceive  all  the  currents  of  the  magnetic  needle  to  be 
replaced  by  a  single  resultant  about  its  equator,  and  all  of  the  ter 
restrial  currents  to  be  replaced  by  a  single  equatorial  current,  then 
that  portion  of  the  latter  current  which  lies  nearest  the  magnet,  will 
attract  the  lower  part  of  the  current  on  the  magnet,  and  repel  that 
on  the  upper  part,  thus  compelling  the  magnet  to  place  itself  in  tho 
meridian. 

What  is  a  solenoid  ?  ( 430.)  What  is  AMPERE  s  theory  o  magnetism  ?  To  what 
did  he  attribute  tho  directive  power  of  tho  earth  ?  Explain  the  action  of  the  ter 
restrial  current  upon  the  magnetic  needle. 


456 


POPULAR    PHYSICS. 


AMPERE  supposes  that  natural  magnets  owe  their  properties  to 
the  long-continued  action  of  electrical  currents.  We  may  suppose 
magnetic  bodies  to  be  made  up  of  atoms,  having  electrical  currents 
flowing  around  them  j  that  is.  of  little  magnets.  These,  when  they 
are  arranged  heterogeneously,  will  exhibit  no  magnetic  properties. 
When  they  are  by  any  action  brought  into  positions  in  which  their 
similar  poles  are  arranged  in  the  same  direction,  they  become 
magnets. 

Galvanometer.  —  Galvanic  Multiplier. 

431.  A  GALVANOMETER  is  an  instrument  for  measuring 
the  force  of  an  electrical 

current.      In  its  simplest  . ^ 

form,  it  consists  of  a  mag 
netic  needle,  «5,  Fig.  296, 
with  a  conducting  wire 
passed  around  it  in  the 
direction  of  its  length. 


Fig.  296. 


When  a  current  of  electri 
city  is  passed  through  the 
wire,  its  presence  will  be  indicated  by  a  motion  of  the  needle,  its 
force  by  the  amount  of  deviation  of  the  needle,  and  the  direction  of 
the  current  will  be  indicated  by  the  direction  towards  which  the 
north  end  of  the  needle  deviates. 

The  GALVANIC  MULTIPLIER  is  a  galvanometer  of  great 
sensitiveness,  but  constructed  on  the  same  principles  as  the 
one  already  described. 

It  is  represented  in  Fig.  297.  It  consists  of  a  copper 
stand,  3/,  supporting  a  glass  cylinder,  as  shown  in  the 
figure.  Under  the  cylinder  is  a  graduated  circle,  beneath 
which  is  a  wooden  frame  wound  with  a  great  number  of 
coils  of  copper  wire.  The  wire  is  insulated  by  being  covered 


How  does  AMPKRK  explain  the  formation  of  natural  'magnets  ?  (431.)  "What 
is  a  Galvanometer?  Describe  it.  Its  action.  What  is  a  Galvanic  Multiplier? 
Describe  it  in  detail. 


GENERAL     PRINCIPLES     OF     ELECTRO-MAGNLriSM. 


457 


with  silk.  The  two  ends  of  the  coil  communicate  with  the 
binding  screws,  m  and  w,  by  means  of  which  they  may  be 
made  to  communicate  with  the  poles  of  a  magnetic  couple. 
A  metallic  frame  supports  a  hook,  from  which  is  suspended 
a  delicate  silken  cord,  s.  This  cord  supports  two  fine  mag 
netic  needles,  the  one,  ab,  above  the  graduated  circle,  and 


Fi;.  297. 

the  other,  .2?,  within  the  coil,  only  a  part  of  which  is  visible 
in  the  figure.  The  two  needles  are  so  united  that  one  can 
not  turn  without  the  other,  and  their  poles  being  placed  in 
opposite  directions,  the  action  of  the  earth  upon  them  is 
completely  neutralized.  Hence  they  are  free  to  obey  the 
least  force. 

Uses   of  the  Galvanic  Multiplier. 

432.     The  Multiplier  is  used  to  indicate  the  feeblest  cur 
rents  of  electricity.     By  means  of  it,  BECQUEREL  established 


(432.)  What  is  the  use  of  the  Multiplier.? 
20 


458  POPULAR  PHYSICS. 

the  fact  that  a  current  is  developed  in  every  chemical  action, 
in  the  imbibition  of  liquids,  and  in  many  other  phenomena. 
By  using  a  galvanometer  with  many  thousands  of  turns  of 
wire,  the  existence  of  electrical  currents  in  animals  and 
vegetables  may  be  demonstrated. 

To  show  the  currents  developed  by  chemical  action,  as,  for  exam 
ple,  the  action  of  acids  upon  metals,  two  fine  platinum  wires  are 
introduced  into  the  binders,  m  and  n.  One  end  of  one  of  the  wires. is 
then  dipped  into  a  glass  of  dilute  sulphuric  acid,  and  the  other  is 
held  in  contact  with  a  plate  of  zinc  which  is  also  dipped  into  the 
dilute  acid.  The  two  needles  which  were  before  parallel  to  oi.  and 
which  we  suppose  to  have  been  placed  in  the  magnetic  meridian, 
immediately  turn  round  and  become  perpendicular  to  the  meridian, 
indicating  the  instantaneous  production  of  a  current.  The  current  in 
this  case  takes  the  direction  indicated  by  the  arrows,  whence  we  con 
clude  that  the  acid  is  positively  and  the  zinc  negatively  electrified. 
This  corresponds  to  what  was  said  on  this  subject  in  a  preceding 
article. 

Magnetizing  by  means   of  an  Electrical  Current. 

433.  If  a  wire  be  wound  around  a  bar  of  iron,  and  a 
current  of  electricity  be  passed  through  the  wire,  it  is  at 
once  converted  into  a  magnet.  The  method  of  making  the 
experiment  is  shown  in  Fig.  298. 


Fig.  298 

If  the  current  cease,  the  iron  bar  at  once  loses  its  mag 
netism.  We  may  in  like  manner  form  a  permanent  magnet 
by  using  a  bar  of  steel  instead  of  a  bar  of  iron. 

Illustrate.    (433.)  How  is  an  iron  bar  converted  into  a  magnet  by  galvanism? 
In  what  way  may  a  bar  of  steel  be  converted  into  a  magnet  ? 


ELECTRO-MAGNETIC    TELEGRAPH. 


459 


The  bar  of  steel  may  also  be  magnetized  by  passing  through  the 
wire  a  spark  from  a  Leydcn  jar.  To  do  this,  one  end  of  the  wire  is 
made  to  touch  the  external  covering  of  the  jar,  and  the  ether  end  is 
brought  into  contact  with  the  button  of  the  jar.  The  steel  bar  is 
magnetized  instantaneously,  thus  showing  the  identity  between  the 
electricity  of  the  galvanic  current  and  that  of  the  Leyden  jar. 


II.  —  ELECTRO-MAGNETIC      TELEGRAPHS.  —  THE       ELECTRO-MOTOR. 

The   Electro-Magnet. 

434.  An  ELECTRO-MAGNET  is  a  magnet  obtained  by  the 
use  of  electricity. 

Electro-magnets  are  generally 
made  of  soft  iron,  bent  in  the 
form  of  a  horse -shoe,  as  shown 
in  Fig.  299.  Upon  each  branch 
is  wound  a  great  number  of 
coils  of  wire,  insulated  by  being 
covered  with  silk.  The  wire  is 
coiled  in  different  directions 
upon  the  two  branches,  and  its 
extremities  are  then  connected 
with  the  poles  of  a  battery. 

In  this  way  magnets  may  be  constructed  of  immense  power,  so 
powerful,  in  fact,  as  to  support  the  weight  of  ten  or  twelve  persons. 
Fig.  300  represents  the  method  of  arranging  the  details  of  a  magnet 
\vhich  is  intended  to  exhibit  a  great  sustaining  power. 

The  plate  in  contact  with  the  two  poles  is  called  an  arma 
ture. 

When  the  instrument  is  of  soft  iron,  it  is  magnetized  instantane 
ously  by  the  passage  of  a  current  of  electricity  through  the  wire,  and 

In  what  other  way  may  it  lie  done  f  What  inference  if>  drawn  from  this  fact? 
(434.)  What  is  an  Electro-Magnet?  How  arc  they  constructed?  What  is  an 
armature  ? 


4-60 


POPULAR    PHYSICS. 


as  instantaneously  loses  its  magnetism  when  the  current  is  stopped, 
or  broken.  This  property  has  been  utilized  in  the  electro-magnetic 
telegraph. 

The   Electrical  Telegraph. 

435.  An  ELECTRICAL  TELEGRAPH  is  an  apparatus  for 
transmitting  intelligence  to  a  distance  by  means  of  electrical 
currents. 

In  1820.  AMPERE  proposed  to  transmit  signals  by  passing  currents 
over  magnetic  needles,  making  use  of  as  many  wires  and  needles  as 

What  in  the  principal  use  of  Ike  electro-magnet  ?  (435.)  "What  is  an  Electrical 
Telegraph  ?  Give  an  outline  of  the  M&tory  of  magnetic  telegraphs. 


ELECTRO-MAGNETIC    TELEGRAPH.  461 

there  are  letters.  In  1837,  STEINHEIL,  of  Munich,  actually  con 
structed  such  a  telegraph. 

In  1831,  Prof.  HENRV,  now  of  the  Smithsonian  Institute,  pub 
lished  the  results  of  his  researches  on  the  subject  of  electro  -mag 
netism,  and  in  subsequent  years,  exhibited  experiments  illustrative 
of  the-ir  application  to  the  transmission  of  power  to  a  distance,  for 
the  purpose  of  producing  telegraphic  effects. 

In  1837,  Prof.  MORSE  invented  a  machine  for  recording  signals 
upon  paper,  and  in  1844,  the  first  working  line  of  telegraph  for 
practical  purposes  was  built  from  Washington  to  Baltimore. 

Many  modifications  of  the  telegraphic  apparatus  have  been  made 
since  its  first  invention.  Three  principal  varieties  are  now  in  use.  all 
of  which  are  based  upon  a  common  principle,  which  is  very  simple. 

At  the  station  from  which  a  telegram  is  dispatched,  is  an  electrical 
battery,  and  at  the  one  where  it  is  to  be  received,  is  an  electro-mag 
net.  The  two  are  connected  by  a  wire  running  between  the  stations. 
When  the  current  is  transmitted  through  the  wire,  the  iron  becomes 
magnetized  and  attracts  an  armature  of  soft  iron,  which  in  turn 
imparts  motion  to  other  pieces,  by  means  of  which  the  signals  are  im 
parted.  When  the  current  ceases,  the  iron  loses  its  magnetism,  and  a 
spring  forces  the  armature  back  to  its  primitive  position.  By  succes 
sively,  breaking  and  restoring  the  current,  the  telegram  is  transmitted. 

•  In  one  form  of  the  telegraph,  the  electro-magnet  causes  a 
needle  to  move  over  a  sort  of  dial,  around  which  are  printed 
the  letters  of  the  alphabet.  The  letter  before  which  the 
needle  stops,  is  the  one  to  be  transmitted.  This  machine 
requires  as  many  signals  as  there  are  letters  in  the  message. 
This  is  the  dial  telegraph. 

In  another  form  of  the  telegraph,  there  are  two  electro 
magnets,  winch  set  in  motion  two  movable  arms  placed  at 
the  extremities  of  a  horizontal  black  line  on  a  white  dial- 
plate.  The  relative  positions  of  the  hands  with  reference  to 
the  fixed  line,  serve  as  conventional  signals,  nearly  in  the 
same  way  as  was  customary  in  the  old-fashioned  telegraph. 
This  is  the  signal  telegraph. 

JIoic  many  Tcinds  are  in  common  use  ?  Explain  their  general  principle.  What 
is  the  dial  telegraph  ?  The  signal  telegraph  ? 


462 


POPULAR    PHYSICS. 


The  dial  telegraph  is  used  in  France  on  the  lines  of  rail 
ways.  The  signal  telegraph  was  used  in  France  for  ordinary 
purposes  until  it  was  replaced  by  MOUSE'S  registering 
apparatus. 

Morse's  Registering  Telegraph. 

436.  In  MORSE'S  telegraph,  the  telegram  is  permanently 
registered  upon  paper  by  means  of  a  conventional  alphabet, 


Fig.  301. 


Fig.  301  represents  the  method  of  dispatching  a  telegram, 
and  Fig.  302  represents  the   method  of  receiving  it.     At 


Where  w«re  these  used?      (436.)  Describe   MORSES  registering  telegraph. 


ELECT  RO-MAGNEHC     TliXEGUAPH.  463 

each  station  the  apparatus  is  identical,  but  it  is  double  ;  that 
is,  composed  of  two  pieces,  the  manipulator  and  receiver. 
These  pieces  are  shown  more  in  detail  in  Figs.  303  and  304. 
In  order  to  explain  the  working  of  this  telegraph,  let  us 
commence  with  Fig.  301. 

Under  the  table  is  shown  the  battery  which  furnishes  the 
electrical  current.  The  current  is  conducted  by  the  wire,  P, 
into  the  manipulator,  which  will  be  described  hereafter. 


Fig.  302. 


From  thence  it  goes  into  a  galvanometer,  g,  which  indicates 
by  a  needle  the  passage  of  the  current ;    it  next  passes 

Explain  the  method  of  working  this  telegraph. 


POPULAR     PHYSICS. 

through  a  piece,  M,  that  serves  as  a  safeguard,  and  from 
thence  reaches  the  wire,  L,  which  passes  to  the  station 
where  the  message  is  to  be  delivered.  We  see  the  same 
wire  entering  at  the  top  of  Fig.  302,  whence  the  current 
passes  through  a  safeguard,  M,  then  into  the  galvanometer, 
from  which  it  goes  to  the  electro-magnet  of  the  receiver. 
After  passing  through  the  electro-magnet,  it  passes  through 
the  wire,  T,  and  is  lost  in  the  earth. 

Morse's   Manipulator  and  Receiver. 

437.  MORSE'S  MANIPULATOR  is  shown  in  Fig.  303.  It 
consists  of  a  wooden  stand,  upon  which  is  a  metallic  lever, 
kh,  turning  about  a  horizontal  axis.  One  end  of  this  lever 
is  raised  up  by  a  spring,  r,  and  the  other  is  traversed  by  a 


Fig.  303. 

stem,  a,  which  rests  upon  a  copper  button,  and  this  in  turn 
communicates  through  the  stand  with  the  wire,  A.  Fig.  303 
represents  the  manipulator  at  the  instant  when  it  receives  a 
dispatch.  The  current  arrives  by  the  wire,  L,  which  is  the 
wire  of  the  line,  rises  into  the  lever,  7jA,  and  descends  by 
the  wire,  A,  to  the  receiver. 

When  it  is  desired  to  transmit  a  signal,  it  becomes 
necessary  for  the  current  from  the  battery,  P,  to  enter  the 
manipulator.  This  is  not  effected  when  the  latter  is  dis 
posed  as  in  Fig.  303,  for  the  lever,  M,  does  not  touch  the 
button  in  which  the  wire,  _P,  terminates.  By  pressing  the 


(437.)  Describe  the  Manipulator  in  detail.     Its  usa 


ELECTRO-MAGNETIC    TELEGRAPH. 


465 


button,  B,  the  lever,  kh,  is  lowered  ;  a  contact  is  established, 
the  current  passes  immediately  into  the  wire,  L,  which  leads 
to  the  other  station. 

The  RECEIVER,  Fig.  304,  is  composed  of  an  electro-mag* 
net,  E,  which,  whenever  a  current  is  transmitted,  acts  by 
attraction  upon  an  armature  of  soft  iron,  m,  fixed  at  the 
extremity  of  a  lever,  win,  and  movable  about  an  axis.  At 
its  extremity,  ??,  the  lever  carries  a  point,  a,  which  may  be 
made  to  press  against  a  movable  fillet  of  paper,  ab.  When 
the  current  does  not  pass  through  the  electro-magnet,  the 
point,  x,  does  not  press  against  the  fillet ;  but  as  soon  as  the 
current  passes,  the  point  is  pressed  against  the  paper,  and 
traces  upon  it  either  a  point,  or  a  line  more  or  less  elongated, 


Fig.  304. 

according  to  the  length  of  time  during  which  the  current  is 
uninterrupted. 

The  fillet  of  paper  is  kept  in  motion  uniformly  by  means 
of  a  train  of  clock-work,  V,  which  turns  the  cylinder,  Z 
(Fig.  301).  The  fillet  of  paper  moving  uniformly  in  the 
direction  from  a  to  5, .the  operator  at  the  other  station,  by 
pressing  the  button  of  the  manipulator,  and  maintaining  the 
pressure  for  greater  or  lesser  periods  of  time,  causes  a  suc 
cession  of  points  and  marks  to  be  made  upon  the  fillet  at 

Describe  tho  Receiver  in  detail    Its  use.    How  is  the  fillet  of  paper  kept  in  moiion  ? 
How  are  the  letters  recorded  ? 


466 


POPULAR     PHYSICS. 


pleasure.  These  marks  are,  by  convention,  mado  to  stand 
for  the  letters  of  the  alphabet,  as  shown  in  the  following 
table 

MORSE'S       ALPHABET. 


a  •  — 

J 

s    •  •  • 

b  

k     .  

t    — 

c  •  •    • 

1   — 

u    •  •  — 

d  

111  

V      .    .    .   —  - 

e  • 

n    —  • 

w   •  

f  

0     • 

X     •   •   • 

o 

P    

y     .   .  .   . 

h  — 

q    

Z      •   •   •       • 

i  .. 

r    •     •  • 

It  only  remains  to  explain  the  PROTECTOR,  M.  Experience 
has  shown  that  the  wires  may,  from  atmospheric  influences, 
accumulate  upon  themselves  sufficient  quantities  of  elec 
tricity  to  prove  troublesome  to  the  operators  of  the  tele 
graph.  The  piece,  J/,  is  destined  to  prevent  any  injurious 
action  of  this  kind.  It  is  composed  of  two  toothed  pieces 
of  metal,  disposed  so  that  the  teeth  are  nearly  in  contact. 
The  current  passes  into  one  of  these  pieces,  whilst  the  other 
is  in  communication  with  the  earth.  If,  from  any  atmos. 
pheric  change,  electricity  accumulates  upon  the  wires  or 
apparatus,  it  is  given  off  by  the  points  to  the  piece  which 
is  in  communication  with  the  earth,  and  shocks  are  thus 
avoided. 

In  what  has  been  said,  only  a  single  wire.  L,  has  been  spoken  of 
as  running  from  station  to  station.  It  would  seem  to  be  necessary, 
in  order  to  complete  the  circuit,  that  a  second  wire  should  be  em* 
p'oyed  ;  such  however  is  not  the  case.  The  employment  of  a  second 
wire  is  avoided  by  connecting  the  two  ends  of  the  single  wire  with 
the  earth.  The  parts  T,  Figs.  30i  and  302,  are  for  this  purpose 


Explain  the  Protector.    Its  use.     Why  is  U  possible  to  operate  a  line  of  telegraph 
with  a  single  wire  f 


ELECTRO-MOTOR.  467 

prolonged  from  the  instruments  till  brought  into  free  communication 
with  the  earth.  The  fluid  then  continues  to  circulate  just  as  though 
a  return  wire  had  been  used. 

Velocity   of  Electricity.— Submarine   Cables. 

438.  It  has  been  found  by  experiment  that  the  velocity 
of  electricity  is  such  as  to  carry  a  current  around  the  earth 
in  about  a  quarter  of  a  second.     For  short  distances,  then, 
we  may  regard  the  transmission  as  instantaneous. 

Since  the  invention  of  the  telegraph,  a  complete  net- work  of  lines 
has  been  established  over  both  continents.  Not  only  have  thousands 
of  miles  of  wires  been  stretched  on  land,  but  submarine  wires  have 
been  laid,  connecting  places  separated  by  hundreds  of  miles  of  water. 
Telegraphic  wires  connect  England  and  Ireland.  England  and  France, 
France  and  Algiers,  and  so  on.  Finally,  an  attempt  has  been  made 
to  connect  the  two  continents,  and  although  it  has  thus  far  failed  to 
be  successful,  there  is  good  reason  to  anticipate  a  complete  success 
at  no  distant  day.  Signals  and  messages  have  been  transmitted 
from  Ireland  to  Newfoundland,  and  the  possibility  of  the  connection 
has  thus  been  fully  demonstrated. 

Electro-Magnetic  Motor. 

439.  Many  attempts  have  been  made,  and  with  partial 
success,  to  employ  electro-magnetism  as  a  motor  for  the 
propulsion  of  machinery.     JACOBT,  of  St.   Petersburg,  con 
structed  an  engine  of  this  kind  in  1838,  which  was  capable 
of  propelling  a  boat  containing  twelve  persons.    Many  other 
machines  have  since   been  constructed,  but  in  all  cases  the 
expense  of  moving  them  has  been  so  great  as  to  preclude 
their  economical  use. 

Fig.  305  represents  an  electro-magnetic  machine,  constructed  ac 
cording  to  the  design  of  M.  FROMENT.  It  is  composed  of  four  elcctro- 

(438)  What  is  the  velocity  of  an  electrical  current?     Give  an  account  of  some, 
of  the,  atibrndr-ine  lint*  nf  tcleg  rap'i.    ( 439.)  Has  electricity  been  used  as  a  motor? 
mdf-Mne  in  detail. 


468 


POPULAR    PHYSICS. 


INDUCTION. 

magnets,  acting  in  pairs  upon  two  pieces  of  soft  iron,  P,  only  one 
of  which  is  seen  in  the  figure.  These  pieces,  attracted  by  the  electro 
magnets.  EF,  transmit  the  motion  by  means  of  a  working-beam,  to 
a  crank,  m,  fixed  at  the  extremity  of  a  horizontal  arbor.  The  latter 
bears  an  iron  fly-wheel,  which  regulates  the  motion.  Finally,  the 
same  arbor  supports  a  piece  of  metal,  n,  of  a  greater  diameter,  the 
use  of  which  will  be  explained  presently. 

The  current  from  the  battery,  P.  entering  A,  passes  into  a  plat 
form  of  cast-iron,  B.  then,  through  different  metallic  pieces,  it 
reaches  the  arbor  and  the  piece  n.  From  thence  the  current  flows 
alternately  to  the  electro-magnets,  EF  and  ef.  The  manner  in 
which  this  alternate  flow  is  effected,  is  shown  in  Fig.  306,  which 
represents  a  section  of  the  piece,  n,  and  its  accessories.  Upon  the 
piece,  n,  is  a  projection,  e,  called  a  cam.  which  in  the  course  of  one 
revolution  touches  successively  two  pallets,  a  and  b\  these  transmit 
to  the  electro-magnets  the  current,  whose  course  is  indicated  by  the 
unfeathered  arrows.  The  feathered  arrows  in  the  figure  show  the 
direction  in  which  the  parts  of  the  machine  move. 

The  current  passing  alternately  into  the  two  pallets,  a  and  6.  and 
thence  into  the  systems  of  electro-magnets,  EF  and  e/,  the  piece.  P, 
is  first  attracted,  and  then  a  similar  piece  at  the  other  extremity  of 
the  arbor  of  the  fly-wheel  is  attracted,  and  so  on.  The  result  is  a 
continuous  rotary  motion,  which  is  transmitted  by  a  driving-band 
to  a  train  of  wheels,  and  so  on  to  the  pumps,  which  it  is  destined  to 
work. 

III.    -INDUCTION.  —  APPLICATION      TO      MEDICINE. 

Induction  by  Currents. 

41O.  We  have  seen  that  the  electricity  of  the  machine 
acts  upon  bodies  by  induction.  The  electricity  of  the 
battery  acts  in  a  similar  manner,  but  only  when  the  cur 
rents  begin  to  flow  and  when  they  cease. 

To  show  this,  take  two  copper  wires,  covered  with  silk,  and  wind 
them  side  by  side  upon  a  bobbin.  Then  fasten  the  two  ends  of  the 

Its  mode  of  action.  (440.)  Does  galvanic  electricity  act  by  induction?  How  is 
this  fihnirn? 


470  POPULAR     PHYSICS. 

first  wire  to  the  two  binders,  m  and  «.  of  the  galvanometer,  Fig.  297. 
Next  connect  one  end  of  the  second  wire  with  one  pole  of  a  feeble 
galvanic  battery.  If  the  other  end  of  the  second  wire  be  brought 
into  contact  with  the  second  pole  of  the  battery,  at  the  instant  of 
contact,  the  needle  of  the  galvanometer  will  indicate  the  production 
of  a  current  in  the  first  wire,  flowing  in  an  opposite  direction  to  that 
of  the  battery.  If  the  contact  is  kept  up,  the  flow  of  the  induced 
current  ceases,  as  is  shown  by  the  needle  of  the  galvanometer  re 
turning  to  its  position  of  rest.  If  the  current  of  the  battery  is  broken, 
the  needle  of  the  galvanometer  is  again  deviated,  but  in  a  contrary 
direction,  indicating  an  induced  current  flowing  in  the  same  direction 
as  that  of  the  battery. 

The  battery  current  is  called  the  inducing  current ;  the 
other  current  is  called  the  induced  one.  These  currents 
conform  to  the  following:  laws  : 

O 

1.  At  the  instant  when   the  inducing  current  begins  to 
flow,  an  induced  current  is  developed  flowing  in  a  contrary 
direction. 

2.  The  inducing  current  continuing  to  flow,  the  induced 
current  ceases. 

3.  At  the  instant  when  the  inducing  current  ceases  to 
flow,  an  induced  current  is   developed  flowing  in  the  same 
direction. 

Properties  of  Induced  Currents. 

441.  Experiment  has  shown  that  induced  currents 
possess  all  the  properties  of  other  electrical  currents.  Like 
them,  they  give  sparks,  produce  violent  shocks,  decompose 
water,  salts,  and  the  like,  and  act  upon  the  magnetic  needle. 

Induced  currents  are  the  more  powerful,  the  longer  the  wires  em- 


Whatisthe  direction  of  the  induced  current  at  the  instant  of  closing  the  cir 
cuit?  Of  breaking  it?  "What  is  the  inducing  curren'.?  The  induced  one?  What 
are  the  laws  that  govern  the  induced  currents  ?  (  441.)  What  are  the  properties  of 
Induced  currents? 


INDUCTION. 


471 


ployed.     Hence,  in  practice  it  is  usual  to  wind  the  wires  upon  bob 
bins,  as  shown  in  Fig.  307. 

The  coil  shown  in  Fig.  307,  consists  first  of  a  cylinder  of  paste 
board,  upon  which  is  wound  about  three  hundred  coils  of  coarse 
copper  wire.  This  is  the  inducing  coil.  Over  it  is  a  finer  wire,  mak 
ing  several  thousand  coils.  These  wires  are  not  only  covered  with 


Fig.  807. 

silk,  but  also  with  an  insulating  varnish  of  gumlac.  At  the  ex 
treme  left  of  the  stand  on  which  the  coil  rests,  are  two  binders 
in  connection  with  the  two  poles  of  a  battery.  From  one  of  them 
proceeds  a  plate  of  copper,  going  to  a  toothed  wheel,  moved  by  clock 
work,  and  communicating  with  one  of  the  ends  of  the  inducing  coil  ; 


How  are  the  wires  wound  ?    How  insulated  ?    How  are  tfo  battery  communi 
cations  made  f 


4:72  POPULAR     PHYSICS. 

the  second  binder  is  in  communication  with  the  other  end  of  the  same 
coil.  The  two  ends  of  the  finer  wire  arc  also  connected  with  binders, 
and  through  them  may  be  connected  with  any  conductor  whatever. 
For  the  purpose  of  administering  a  shock,  the  binders  are  provided 
with  wires  having  copper  handles,  which  are  to  be  grasped,  as  shown 
in  the  figure. 

When  the  instrument  is  in  operation,  the  current  from  the  battery 
is  continually  broken  by  means  of  the  toothed  wheel,  and  there 
results  a  succession  of  shocks,  two  at  each  interruption  of  the  cur 
rent.  The  shocks  that  arise  at  the  beginning  of  the  flow  are  almost 
nothing,  whilst  those  which  take  place  at  the  time  of  interruption 
are  quite  severe. 

The  principle  of  induced  currents  has  been  applied  in  construct 
ing  instruments  for  generating  large  quantities  of  electricity.  The 
induction  coil,  as  improved  by  Ruhmkorff,  is  one  of  the  most  re 
markable  instruments  of  this  class.  In  some  of  his  larger  instru 
ments  he  uses  more  than  sixty  miles  of  wire  in  the  secondary  coil. 


Physiological  effects  of  Electrical  Currents. 

442.  Electrical  currents  have  been  employed  in  the 
treatment  of  certain  diseases,  especially  those  connected 
with  the  nervous  system.  Electricity  has  a  powerful  action 
upon  the  animal  economy,  and  when  judiciously  applied 
possesses  considerable  curative  power. 

Fig.  308  represents  one  of  the  many  forms  that  have  been  given 
to  the  electrical  apparatus,  for  the  purpose  of  acting  upon  the  human 
body.  It  consists  of  a  wooden  box,  upon  which  is  mounted  a  copper 
cylinder,  inclosing  a  bobbin  of  two  wires.  The  box  has  a  drawer  of 
zinc,  in  which  is  a  small  quantity  of  salt  water.  A  plate  of  well 
calcined  carbon,  impregnated  with  nitric  acid,  is  plunged  into  this 
solution.  In  a  word,  the  combination  constitutes  a  modified  form  of 
a  BUNSEN'S  couple.  Two  copper  slips  communicate,  the  one  with 
the  zinc  and  the  other  with  the  carbon,  conducting  the  current  to  the 


How  are  shocks  given  ?  What  arrangement  is  made  for  continually  breaking 
the  current?  How  may  the  ahocks  l>e  increased?  ^  442.)  What  application  has 
been  made  to  medicine  ?  Explain  Fig.  808. 


APPLICATION    TO     MEDICINE. 


473 


large  wire  of  the  coil,  through  a  piece  of  machinery  for  breaking  the 
current.  This  current-breaker  consists  of  a  small  plate  of  soft  iron, 
attracted  by  an  electro-magnet  in  the  centre  of  the  bobbin.  It  is 
attracted  when  the  current  passes,  and  immediately  interrupts,  or 
breaks  it.  The  induced  current  is  conducted  by  wires  to  two  sponges 


Fig.  80S. 

saturated  with  salt  water,  or  fresh,  according  as  it  is  desired  to  make 
a  more  or  less  intimate  communication  with  the  part  through  which 
the  shocks  are  to  be  passed.  Finally,  the  method  of  applying  the 
shocks  is  shown  in  Fig.  308. 

Electrical  Fishes. 

443.  Certain  fishes  possess  the  power  of  imparting  a  shock  that 
compares  in  intensity  with  that  of  a  powerful  Leyden  jar.  SucJi 
fishes  are  called  electrical  fishes,  and  are  of  three  kinds,  the  most 


(  443.)  Describe  the  electrical  fishes. 


474:  POPULAR     PHYSICS. 

interesting  of  which  is  the  electrical  eel  of  South  America.  This 
fish  was  studied  by  HUMBOLDT  and  BONPLAND,  who  have  given  a  com 
plete  description  of  it. 

The  shocks  given  by  electrical  fishes  are  due  to  electricity 
generated  in  the  body  of  the  fish.  MATTEUCI  showed  that  sparks 
could  be  obtained  from  the  fish,  and  also  that  the  galvanometer  is 
affected  when  one  of  its  wires  is  brought  into  connection  with  the 
back  of  the  fish,  and  the  other  with  its  belly. 

Tn  all  cases  the  shock  is  voluntary,  and  serves  as  a  means  of 
defense  against  enemies. 


To  what  are  their  shocks  due  f     What  observations  were  made  by  MATTEUCI  ? 


CHAPTER  XL 

APPLICATION    OP   PHYSICAL   PRINCIPLES   TO   MACHINES. 
!.  —  GENEBAL      PRINCIPLES. 

Definition  of  a  Machine. 

414.  A  MACHINE  is  a  contrivance  by  means  of  which  a 
force  applied  at  one  point,  is  made  to  produce  an  effect  at 
some  other  point. 

The  force  applied  is  called  the  power,  and  the  force  to  be  overcome 
is  called  the  resistance. 

Motors. 

445.  The  working  of  a   machine  requires  a  continued 
application  of  power.    The  source  of  this  power  is  called 
the  MOTOK. 

Some  of  the  most  important  motors  are  muscular  effort,  as  exerted 
by  man  or  beast,  in  various  kinds  of  work ;  the  weight  and  impulse 
of  water,  as  in  water-mills  ;  the  impulse  of  air,  as  in  wind-mills  ;  the 
elastic  force  of  springs,  as  in  watches;  the  expansive  force  of  vapors 
and  gases,  as  in  steam  and  hot-air  engines.  The  last  is,  perhaps, 
the  most  useful  of  the  motors  mentioned. 

Object  and  Utility  of  Machines. 

446.  The  object  of  a  machine  is  to  transmit  the  power 
furnished  by  the  motor,  and  to  modify  its  action  in  such  a 
manner  as  to  cause  it  to  produce  a  useful  effect. 

(444.)  What  is  a  machine  ?  The  power?  The  resistance  f  (445.)  What  is  a  mo 
tor?  Mention  some  of  the  most  important.  (446.)  What  is  the  object  of  a  machine  I 


476  POPTTLAH   PHYSICS. 

In  no  case  does  a  machine  add  anything  to  the  power  applied  to  it-, 
on  the  contrary,  it  absorbs  more  or  less  of  this  power,  according  to 
tho  nature  of  the  work  to  be  done  and  the  connection  existing 
between  the  parts. 

Some  of  the  circumstances  which  cause  an  absorption  of  power 
are  the  rubbing  of  one  part  upon  another,  the  stiffness  of  bands  and 
belts,  the  resistance  of  the  air,  the  adhesion  of  one  part  to  another, 
and  the  want  of  hardness  and  elasticity  in  the  materials  of  which  the 
machine  is  constructed.  The  resistances  arising  from  these  causes  are 
called  hurtful  resistances.  They  not  only  absorb  much  of  the  power- 
applied,  but  they  also  contribute  to  wear  out  the  machine.  The 
existence  of  these  resistances  in  every  machine  requires  a  continued 
supply  of  power  to  overcome  them,  in  addition  to  that  necessary  to 
perform  tho  useful  work.  Hence  the  absurdity  of  attempting  to 
obtain  perpetual  motion. 

Quantity  of  Work  of  a  Force. 

447.  The  idea   of  WORK,  in  mechanics,  implies  that  a 
force  is  continually  exerted,  and  that  the  point  at  which  it  is 
applied  moves  through  a  certain  space.     Thus,  in  raising  a 
weight,  the  work  performed  depends  first  upon  the  weight 
raised^  and  secondly  upon  the  height  through  which  it  is 
raised.     The*  quantity  of  work  of  a  force  in  any  given  time, 
is  measured   by  the   intensity   of  the  force,  expressed  in 
pounds,  multiplied  by  the  distance  through  which  it  is  ex 
erted,  expressed  in  feet.     This  distance  is  called  the  path 
described. 

Equilibrium  of  a  Machine. 

448.  A  machine  is  in  EQUILIBRIUM  when  the  power  and 
resistance  exactly  balance  each  other, 

In  determining  the  circumstances  of  equilibrium,  it  is  customary  to 
neglect  the  hurtful  resistances  in  the  first  approximation,  and  then  to 


Can  a  machine  create  pmcer  f  What  are  hurtful  resistances?  Their  effect  1 
(44r7.)  What  is  meant  by  work  ?  Illustrate.  What  i*  the  measure  of  the  quantity 
of  work  ?  (44:8.)  When  is  a  machine  In  equilibrium  ?  What  is  the  condition  oj 
equilibrium  when  the  hurtful  resistances  are  neglected? 


ELEMENTARY    MACHINES.  4:77 

take  account  of  them  as  corrections.  If  the  hurtful  resistances  be 
neglected,  it  will  be  found  that  any  machine,  working  uniformly,  is 
in  equilibrium,  when  in  any  given  time  the  quantity  of  work  of  the 
power  is  equal  to  that  of  the  resistance. 


II. —  ELEMENTARY      MACHINES. 

Mechanical  Powers. 

449.  The  elementary  machines  are  seven  in  number,  viz., 
the  cord;  the  lever;  the  inclined  plane  ;  the  pulley;  the  whed 
and  axle;  the  screw ;   and  the  wedge.     These   seven   are 
called  mechanical  powers.     The  first  three  are  simple  ele 
ments  ;  the  remaining  ones  are  combinations  of  these  three. 

The  principles  of  the  lever  and  inclined  plane,  so  far  as  necessary 
to  an  understanding  of  the  principles  of  Physics,  have  already  been 
explained  in  Chapter  I.  In  the  following  articles  those  principles  are 
repeated,  in  connection  with  a  description  of  the  other  mechanical 
powers.  In  the  cuts  which  follow,  the  power  and  resistance  are 
represented  by  arrow-heads,  the  former  being  denoted  by  the  letter 
P,  and  the  latter  by  E. 

The  Cord. 

450.  CORDS,  and  BANDS  or  BELTS,  are  used  for  transmit 
ting  motion  from  one  point  to  another,  as  in  the  pulley. 
Chains  are  often  employed  for  the  same  purpose,  as  in  the 
watch. 

Cords,  belts,  and  chains,  should  be  as  flexible  as  is  consistent  with 
sufficient  strength. 

The  Lever. 

451.  A  LEVER  is  an  inflexible  bar,  free  to  turn  about  an 
a  xis.     This  axis  is  called  the  FULCRUM.     (See  Arts.  30,  3 1 ,  and 
32.)     Levers  may  be  either  straight  or  curved.      The  dis- 

(449.)  How  many  mechanical  powers  are  there  ?  Name  them.  (450.)  "What  la 
the  use  of  a  cord  or  band  in  machinery  ?  (451.)  What  is  a  lever  ? 


478 


POPULAR    PIIYSICS. 


tances  from   the  fulcrum  to  the  lines  of  direction  of  the 
B          power  and  resistance  are  called  lever 
arms. 

In  the  lever,  MN,  F  is  the  fulcrum,  MP 
and  NR  are  the  lines  of  direction  of  the 
power  and  resistance,  FA  is  the  lever  arm 
of  the  power,  and  FB  is  the  lever  arm  of 
Fig.  309.  £|ie  resistance. 

Levers  are  divided  into  three  classes: 

In  the  first  class  (Fig.  310),  the  fulcrum  is  between  the  power  and 
the  resistance.     The  steelyard  is  a  lever  of  this  class. 

In  the  second  'class  (Fig.  311),  the  resistance  is  between  the  power 
and  fulcrum.     The  rudder  of  a  ship  is  a  lever  of  this  class. 

In  the  third  class  (Fig.  312),  the  power  is  between  the  resistance 
and  the  fulcrum.     The  treadle  of  a  lathe  is  a  lever  of  this  class. 


f 


Kf 

Fig.  811. 


J 


Fig.  810.  Fig.  811.  Fig.  812. 

In  all  cases  the  paths  described  by  the  points  of  application  of  the 
power  and  resistance  will  be  proportional  to  their  lever  arms,  and 
when  in  equilibrium,  the  power  will  be  to  the  resistance  as  the  lever 
arm  of  the  resistance  is  to  the  lever  arm  of  the  power. 

Compound  Levers. 

452.  A  COMPOUND  LEVER  is  a  combination  of  simple  levers,  sc 
arranged  that  the  resistance  in  one,  acts  as  a  power  in  the  next, 
throughout  the  combination. 

Compound  levers  are  used  for  magnifying  small  motions,  as  in 
showing  the  expansion  of  bodies ;  or  to  enable  a  small  weight  to 
balance  a  large  one,  as  in  the  hay-scale  and  in  other  weighing 
machines.  The  principle  of  the  compound  lever  applies  in  trains  of 
wheelwork. 


Fulcrum?    Lever  arms?    Illustra**.    How  many  clashes  of  levers  are,  there  t    De 
scribe  each  claw,  and  illustrate.    (4t52.)   What  is  a  compound  lever  t    Its  uses  ? 


ELEMENTAKV    MACHINES. 


479 


The  Inclined  Plane. 

453.     An  INCLINED  PLANE  is   a  plane  inclined  to  the 

horizon.     (See  Arts.  49,  50,  and  51.) 

In    machinery,  the    inclined    plane    is   seldom   used,  except    in 
ornbination. 


The  Pulley. 

454.  A  PULLEY  is  a  wheel  free  to  turn  about  on  axis 
and  having  a  groove  around  it  to  receive  a  cord.     The  axis 
turns  in  a  frame  called  a  block. 

A  pulley  is  said  to  be  fixed  or  movable,  according  as  its  block  is 
fixed  or  movable. 

Single  Fixed  Pulley. 

455.  In  this  pulley  the  block,  O,  is  fixed,  and 
the  wheel,  AB,  turns  within  it.     The  effect  of  the 
fixed  pulley  is  simply  to  change  the  direction  of  a 
force. 

Single  Movable  Pulley. 

456.  In  this  pulley  the  block,  O,  is  movable,  and  the 
wheel  turns  within  it.  C 


Pulleys  are  combinations  of  the  cord  and  lever.  In 
the  fixed  pulley  we  may  regard  AB  as  a  lever,  whose 
lever  arms  are  OA  and  OB,  and  whose  fulcrum  is  O. 
In  the  movable  pulley  we  may  regard  AB  as  a  lever  of 
the  second  class,  whose  fulcrum  is  A,  and  whose  lever 
arms  are  AB  and  AO. 

Pulleys  are  used  for  raising  weights,  for  working  the 
rigging  of  ships,  and  the  like.  They  are  frequently  used 
in  combinations. 


(453.)  What  is  an  inclined  plane?  (454.)  What  is  a  pulley?  A  block?  When 
flx*d  f  Movable  t  (455.)  Describe  the  single  fixed  pnlley.  Its  effect.  (456.)  De- 
•cribe  the  single  movable  pulley.  Of  what  simple  machines  are  pulleys  composed  f 


480 


POPULAR    PHYSICS. 


Combinations  of  Pulleys. 

457.  Figure  315  represents  a  combination  of  three  movable 
pulleys,  in  which  there  is  a  separate  cord  for  each  pulley.  One  end 
of  each  cord  is  attached  to  a  fixed  sup 
port,  the  other  end  being  attached  to  the 
block  of  the  pulley  next  in  order,  except 
the  last  one,  at  the  free  extremity  of 
which  the  power  is  applied.  The  re 
sistance,  R,  is  applied  to  the  block  of  the 
first  pulley.  This  combination  is  diffi 
cult  to  use,  and  occupies  a  great  deal  of 
space. 

Figure  316  represents  a  combination 
of  fixed  and  movable  pulleys,  the  former 
in  one  block  and  the  latter  in  another. 
A  single  cord  is  employed,  having  one 
end  made  fast  to  the  fixed    block,    and    then     passing 
around  the   wheels,  alternating  between  those  of   the      Fi«-816- 
movable  and  those  of  the  fixed  block.     The  power  is  applied  at  the 
free  extremity  of  the  cord,  and  the  resistance  to  the  movable  block. 
The  pulleys  in  each  block,  instead  of  being  one  above  another,  as  in 
the  figure,  are  often  placed  side  by  side. 


The  Wheel  and  Axle. 

458.  The  WHEEL  AND  AXLE  consists  of  a  wheel,  or  drum, 
A,  mounted  upon  an  axle,  B.  The 
power  is  applied  at  one  extremity  of  a 
cord  wrapped  around  the  wheel,  and 
the  resistance  at  one  extremity  of  a 
second  cord  wrapped  around  the  axle 
in  a  contrary  direction.  The  whole  is 
supported  on  a  suitable  frame,  by 
means  of  pivots  projecting  from  the 
ends  of  the  axle. 


(£57.)  Describe  the  combination  of  putteys  with  separate  cords.     With  a  single 
nnrd.    (458.)  What  ia  a  wheel  and  axle  ? 


ELEMENTARY    MACHINES. 


481 


The  principle  of  the  wheel  and  axle  is  similar  to  that  of  the  pulley, 
and  it  is,  like  that  machine,  a  combination  of  the  cord  and  the  lever. 
In  machinery  its  principal  use  is  in  transmitting  motion  of  rotation 
from  one  piece  to  another. 

The  Windlass  and  Capstan. 

459.  The  WINDLASS  consists 
of  an  axle,  or  arbor,  AB,  and 
a  crank,  BCD,  by  means  of 
which  it  is  turned.  The  crank 
consists  of  an  arm,  BO,  perpen 
dicular  to  the  axle,  called  the 
crank  arm,  and  a  second  arm, 
DO,  perpendicular  to  the  first, 
called  the  crank  handle.  The 
power  is  applied  to  the  crank 
The 


Fig.  818. 


handle,  and  the  resistance  to  a  rope  wrapped  around  the  axle, 
windlass  is  principally  used  in  raising  weights. 

The  capstan  differs  from  the  windlass  in  having  its  axis  vertical, 
and  in  being  turned  by  means  of  levers  inserted  in  holes  made  in  the 
head  of  the  axle,  instead  of  by  a  crank.  It  is  much  used  on  ship 
board  for  raising  anchors  and  the  like. 

The  Differential  Windlass. 

460.  This  differs  from  the 
common  windlass,  in  having  an 
axle  formed  by  two  drums,  A 
and  B,  of  different  diameters. 
A  cord  is  attached  to  the  larger 
cylinder  and  wrapped  several 
times  around  it,  after  which  it 
passes  under  a  movable  pulley, 
C,  and  is  then  wrapped  in  a 
contrary  direction  around  the 
smaller  cylinder.  The  power  is 
applied  to  the  crank  arm,  and 


Pig.  81$. 


the  resistance  to  the  block  of  the  movable  pulley. 


Its  use  in  machinery  ?    (459.)  What  is  a  windlass  T     Describe  it.     What 
crank  T    What  is  a  capstan  f     (4:60.)  Deserve  the  differential  windlass. 


4-82 


POPULAR   PHYSICS. 


When  the  cord  is  wound  upon  the  larger  cylinder,  it  unwinds  from 
the  smaller  one,  but  in  a  less  amount,  so  that  the  totd  effect  is  to  raise 
the  weight,  E.  The  power  of  this  machine  may  be  increased  by 
making  the  two  drums  nearly  equal  in  diameter. 


The  Screw. 

461.  The  SCREW  is  essentially  a  combination 
of  inclined  planes.  It  consists  of  a  colid  cylinder, 
enveloped  by  a  spiral  projection  called  the  thread. 
The  two  faces  of  the  thread  are  nothing  more 
than  inclined  planes  wound  around  the  cylinder  of 
the  screw, 
rig.  820.  The  gcrew  workg  into  a  go]id^  fitfct;d  ^  rece;ve 

it,  called  the  nut.  The  nut  may  be  fixed,  the  screw  turning 
within  it,  or  the  screw  may  be  fixed,  the  nut  turning  upon  it. 
Motion  is  imparted  to  the  one  or  the  other,  as  the  case  may 
be,  by  means  of  a  lever,  at  the  extremity  of  which  the  power 
is  applied. 

The  endless  screw  is  a  screw  secured  by  shoulders,  so  that  it  can 
not  move  in  the  direction  of  its  length, 
.and  working  into  a  toothed  wheel.  When 
the  screw  is  turned,  it  imparts  motion  to 
the  wheel,  which,  in  turn,  may  be  made  to 
move  a  train  of  wheelwork. 

Machines  of  this  kind  are  used  in  regis 
tering  the  number  of  turns  of  an  axle,  as, 
for  example,  the  shaft  of  a  steamboat.  An 
endless  screw  is  arranged  so  as  to  turn  as 
many  times  as  the  shaft,  and  is  connected 
with  a  train  of  light  wheelwork.  The 
wheels  bear  indices,  by  means  of  which 
Fig.  821.  tke  number  Of  turns,  in  any  given  time, 

may  be  read  off.     This  .arrangement  is  extensively  used  in  gas  and 
water-meters,  and  also  in  various  branches  of  manufacture. 


Its  use.    (461.)  What  is  a  bcrew?    Its  thread?    Its  nut?    Describe  the  endless 
"?v?r.    Its  uses. 


HURTFUL   RESISTANCES.  483 

The  "Wedge. 

462.     The  WEDGE  is  a  solid,  bounded  by  a  rectangle,  BD, 
called  the  back;   two  rectangles,  AF  and 
DF,  called  faces,  and  two  triangles,  ADE 
and  BCF,  called  ends.    The  line,  EF,  in 
which  the  faces  meet,  is  called  the  edge. 

The  wedge  is  a  combination  of  two  in 
clined  planes,  and  is  used  in  splitting  and 
cutting  instruments.  The  power  is  applied 
to  the  back,  and  may  consist  either  of  a 
blow  or  of  a  steady  pressure.  The  resist-  y\g.  822. 
ance  is  applied  to  the  faces. 

The  power  of  a  wedge  may  be  increased,  by  increasing  the  length 
of  its  faces,  and  by  diminishing  the  breadth  of  its  back. 


III. HURTFUL      RESISTANCES. 

Friction. 

463.  FKICTION  is  the  resistance  which  one  body  ex- 
periences  in  moving  upon  another  when  the  two  bodies  are 
pressed  together.  This  resistance  arises  from  inequalities  in 
the  surfaces,  the  projections  of  the  one  sinking  into  the  de 
pressions  of  the  other.  To  overcome  the  resistance  thus 
produced,  a  force  must  be  applied  sufficient  to  break  off  or 
bend  down  the  projecting  points,  or  else  to  lift  the  moving 
body  over  the  inequalities. 

Friction  is  distinguished  as  sliding  and  rolling.  The 
former  arises  when  one  body  is  drawn  upon  another ;  the 
latter,  when  one  body  is  rolled  upon  another.  Everything 
else  being  equal,  the  former  is  greater  than  the  latter. 

It  has  been  found  by  experiment  that  the  sliding  friction,  between 
the  same  two  bodies,  is  proportional  to  the  force  with  which  they 

(4G3.)  What  is  a  wedge  ?  Of  what  is  it  compounded  ?  Its  use  ?  How  may  its 
power  Z>«  increased  ?  (463.)  What  is  friction  ?  How  is  it  caused  ?  Distinction  be- 
tween  sliding  and  rolling  friction  ?  Which  is  the  greater  ?  Explain  the  laws  of  &ott 
eliding  and  rolling  friction. 


484  POPULAR  PHYSIOS. 

are  pressed  together,  and  independent  of  the  extent  of  surface  in 
contact. 

It  has  been  found,  when  a  wheel  or  pivot  rolls  upon  a  surface,  that 
the  rolling  friction  is  proportional  to  the  pressure,  and  inversely  pro 
portional  to  the  diameter  of  the  wheel  or  pivot. 

In  many  cases  there  is  a  combination  of  sliding  and  rolling  friction. 
In  the  case  of  a  car  on  a  railroad,  the  friction  at  the  axle  is  sliding, 
while  that  at  the  track  is  rolling. 

Sliding  friction  may  be  greatly  diminished  by  interposing  between 
the  rubbing  surfaces  unguents,  such  as  lard,  tallow,  oil,  and  various 
compositions. 

For  slow  motions  and  great  pressures  the  more  consistent  unguents, 
as  lard,  tallow,  and  the  like,  are  used ;  for  rapid  motions  and  light 
pressures,  oDs'are  generally  employed. 

Stiffness  of  Cords. 

464.  When  a  cord  is  wound  upon  a  wheel  or  axle,  a 
certain  amount  of  force  is  required  to  bend  it.     The  re 
sistance  which  the  cord  thus  offers  to  bending  is  classed  as  a 
hurtful  resistance.     This  resistance  should  be  obviated,  as  far 
as   possible,  by  selecting  bands   and   cords,  wrhich   are   as 
flexible  as  is  consistent  with  due  strength. 

Atmospheric  Resistance. 

465.  The  atmosphere  exerts  a  powerful  resistance  to  the 
motion  of  bodies  moving  through  it.     It  has  been  found, 
both  by  theory  and  experiment,  that  this  resistance  is  pro 
portional  to  the  greatest  cross  section  of  the  body,  made  by 
a  plane  perpendicular  to  the  direction  of  the  motion,  and 
also  to  the  square  of  the  body's  velocity.     To  obviate  this 
resistance,  as  far  as  possible,  the  pieces  which  have  a  rapid 
motion  should  have  as  small  a  cross  section  as  is  consistent 
with  due  strength. 


Explain  the  two  kinds  of  friction  in  a  car.  How  is  ft-iction  diminished  ?  When 
would  you  employ  consistent  and  when  fluid  unguents?  (£64:.)  How  does  the  stiff- 
ness  of  cords  produce  resistance  ?  How  lessened  ?  (465.)  Explain  the  subject  of 
atmospheric  resistance.  How  lessened  ? 


WUEELWOKK.  485 

The  principle  of  atmospheric  resistance  has  been  employed  to 
regulate  the  velocity  of  certain"  machines.  To  effect  this  object,  a 
wheel  is  arranged  bearing  tanes,  which,  striking  against  the  air 
generate  a  resistance  that  prevents  the  velocity  from  passing  a  certain 
limit.  It  is  this  principle  which  renders  the  parachute  a  safe  means 
of  descending  from  a  balloon. 


IV.  — WHEELWOEX. 

Trains  of  Wheels. 

466.  The  power  furnished  by  the  motor  of  a  complex 
machine,   is   usually  transmitted  through   a   succession  of 
pieces,  to   the   working    point.      These  connecting   pieces 
are,   in    general,   wheels    and    axles,   and   taken    together 
they  form  what  is  called  a  train.     A  wheel  which  imparts 
motion  to  a  succeeding  one,  is  called  the  driver  /  that  to 
which  motion  is  imparted,  is  called  the  follower. 

Mode  of  Connection. 

467.  There  are  various  methods  by  means  of  which  one 
wheel  may  be  made  to  act  upon  another. 

First.  By  simple  contact.  The  driver, 
A,  being  slightly  pressed  against  the  fol 
lower,  B,  the  friction  between  the  wheels 
is  sufficient  to  impart  a  motion  of  rotation 
from  the  former  to  the  latter. 

To  increase  the  friction  and  avoid  sliding,  the  surfaces  are  fre 
quently  covered  with  soft  leather.  In  all  cases  the  motion  of  the 
follower  is  in  a  contrary  sense  to  that  of  the  driver,  as  indicated  by 
the  arrows. 

Secondly.  By  means  of  bands  or  belts.  The  band  is 
passed  around  the  circumferences  of  both  wheels,  and  when 

What  -MS9  has  been  made  of  atmospheric,  resistance  as  a  regulator  t  (4G6.)  What 
fb  a  train  ?  A  driver?  A  follower  ?  (467.)  Explain  the  mode  of  connection  by  sim 
ple  contact.  IIow  is  sliding  avoided  t 


486 


POrtJLAK    PHYSICS. 


tightened,  a  sufficient  amount  of  friction  is  produced  to  im 
part  motion  from  the  driver  to  the  follower. 

When  the  band  does  not  cross  between  the  wheels,  they  both  re 
volve  m  the  same  direction,  as  indicated  in  Fig.  324.  "When  the 
band  crosses  between  the  wheels,  they  revolve  in  opposite  directions, 


Fig.  324. 


Fig.  825. 


as  indicated  in  Fig.  325.  Belts  are  made  of  leather,  gutta  percha, 
and  the  like.  They  are  flat  and  thin,  and  the  drums  on  which  they 
run  should  be  slightly  elevated  toward  the  middle  of  their  thickness. 
Cords  are  made  of  catgut,  hempen  fibres,  or  wire,  nearly  cylindrical. 
The  drums,  or  pulleys,  on  which  they  run  should  be  elevated  at  the 
edges.  Chains  are  also  used,  and  in  this  case  the  drums  should  be 
grooved,  and  either  notched  or  toothed,  so  as  to  fit  the  links  of  the 
chain. 

Thirdly.  By  means  of  projections  on  the  circumferences  of 
the  wheels  called  teeth. 

A  small  wheel,  C,  mounted  on  the  axle  of  a  large  one,  B,  is  called 
ft  pinion,  and  its  projections  are  called 
leaves.  In  the  figure  the  teeth  of  the 
wheel  A  engage  with  the  leaves  of  the 
pinion  C,  and  the  teeth  of  the  wheel  B 
engage  with  the  leaves  of  the  pinion  D. 
If  the  wheel  A  is  turned  in  the  direction 
indicated  by  the  arrow,  the  wheel  B  will 
revolve  in  a  contrary  direction,  and  the 
wheel  F  in  the  same  direction.  A  wheel 
whose  teeth  project  from  its  circumfer 
ence,  as  shown  in  Fig.  326,  is  called  a 
spur-wlieel. 

Explain  the  mode  of  connection  by  bands  and  belts.  Explain  the  two  methods  of 
arranging  the  bands.  The  kind  oficheel  required  fir  "belts.  For  cords,  for  chains. 
Explain  the  mode  of  connection  ~by  teeth.  What  is  a  pinion  t  Leaves  t  Illustrate, 
What  is  a  epur-wheelf 


REGULATORS.  4:87 

111  the  combination  shown  in  Fig.  326,  the  axes  of  the  wheels  are 
supposed  to  be  parallel.  When  the  axes  are  not  parallel  to  each 
other,  motion  of  rotation  may  be  communicated  by  means  of  beveled 
wheels,  as  shown  in  Fig.  13.  If  the  axes  are  perpendicular  to  each 
other,  beveled  wheels  may  be  used,  as  in  Fig.  13,  or  the  driver  may 
have  its  teeth  set  perpendicular  to  its  face  and  working  into  a 
pinion.  Such  a  wheel  is  called  a  crown-wheel. 

When  the  number  of  teeth  of  the  driver  is  greater  than  that  of  the 
follower,  the  angular  velocity  of  the  latter  will  be  greater  than  that 
of  the  former.  By  a  suitable  adjustment  of  the  number  of  teeth  on 
the  different  wheels,  the  angular  velocity  may  be  multiplied  at 
pleasure. 


V.  —  EEGTJLATOES. 

The  Governor. 

408.  The  GOVERNOR  is  a  contrivance  for  regulating  the 
supply  of  motive  force.  One  form  of  this  contrivance  is 
shown  in  Fig.  327. 

AB  is  a  vertical  axis,  connected  with  the  machine  near  its  work 
ing  point,  and  revolving  with  a  velocity  pro 
portional  to  that  of  the  working  point ;  FE 
and  GD  are  arms  turning  with  the  axis,  and 
bearing  heavy  balls,  D  and  E,  at  their  ex 
tremities ;  the  arms  are  attached,  by  hinge 
joints,  at  G  and  F  to  two  bars,  OG  and  OF, 
and  these  bars  are  connected  by  hinge  joints 
with  the  axis  at  0.  The  arms  FE  and  GD 
are  also  connected  by  hinge  joints,  with  a 
ring,  H,  which  is  free  to  slide  up  and  down 
the  axis,  AB.  Fig.  327. 

When  the  axis  revolves,  the  centrifugal  force  developed  in  the  balls 
causes  them  to  recede  from  AB,  and  depresses  the  ring,  H.  This 
causes  the  lever,  BK,  to  turn  about  its  fulcrum,  K,  and  when  the 
velocity  has  become  sufficiently  great,  the  lever  operates  to  close  a 


A  leveled  wJieel  ?  A  crown  wJieel  f  When  is  the  angular  motion  of  the  follows- 
greater  than  that  of  the  driver  f  (4:68.)  What  is  a  governor  ?  Describe  that  show 
in  thejigure^  and  explain  its  action. 


488  roruLAK  PHYSICS. 

valve,  and  shut  off  the  motive  power.  "When  the  velocity  again 
diminishes,  the  balls  approach  the  axis,  the  ring,  II,  rises,  and  the 
valve  is  opened.  The  governor  may  be  adjusted  so  as  to  secure  any 
desirable  velocity  at  the  working  point. 


The  Fly- Wheel 

469.     The    FLY-WHEEL  is    a  contrivance   for    obviating 
irregularities  of  motion  in  a  machine. 

It  usually  consists  of  a  heavy  rim  of  iron  connected  with  the  axle 
by  radial  arms,  as  shown  in  the  figure.  It 
is  mounted  on  an  axle  near  the  working  point 
of  the  machine,  and  when  the  motive  power 
exceeds  the  amount  required  to  do  the  work, 
the  excess  goes  to  overcome  the  inertia  of 
the  fly-wheel,  with  but  a  slight  increase  of 
its  angular  velocity.  On  the  other  hand, 
when  the  motive  power  is  less  than  that  re 
quired  to  do  the  work,  the  fly-wheel  acts  by 
its  inertia,  giving  out  the  force  stored  up  in 

it,  with  but  a  slight  decrease  of  angular  velocity,  and  thus  supplies 
the  deficiency.  By  a  proper  adaptation  any  desired  uniformity  of 
motion  may  bo  attained. 


VI. — PRIME       ttOVEEB. 

Definition  of  a  Prime  Mover. 

47O.  A  PRIME  MOVEK  is  a  piece,  or  combination  of  pieces, 
to  which  the  force  of  the  motor  is  immediately  applied,  and 
from  which  that  force  is  transmitted  through  the  connecting 
pieces  to  the  working  point.  The  most  important  prim  a 
movers  are  water-wheels  and  steam-engines. 


(469.)  What  is  a  fly-wheel  ?    Describe  it  and  its  action.    (470.)  Woat  is  a  prime 
mover  ?    What  are  the  most  important  ones  ? 


PRIME   MOVERS. 


489 


Fig.  329. 


Water-Wheels. 

A  WATER-WHEEL  is  a  wheel  put  in  motion  by  the  action 
of  water.  "Water-wheels  are  divided  into  two  classes  —  verti 
cal  and  horizontal  wheels.  There  are  three  principal  varieties 
of  vertical  wheels  —  overshot,  undershot,  and  breast  icheels. 
The  most  important  of  the  horizontal  wheels  is  the  turbine, 

The  overshot  wheel  consists  of  a  cylindrical 

drum,  A,  terminated  at  its  ends  by  projecting 

rings,  B,  called  crowns.    The  space  between 

the  crowns  is  divided  into  cells,  by  curved 

or  angular  partitions  ;  these  cells  are  called 

"buckets,  and  they  are  constructed  so  as  to 

retain  the  water  as  long  as  possible.     The 

water  is  delivered  by  a  sluice,  0,  near  the  top 

of  the  wheel,  and  acting  by  its  weight,  it 

imparts  a  rotary  motion  to  the  wheel. 

The  undershot  wheel  resembles  the  overshot  wheel  in  its  general 

construction,  but  differs  from  it  in 
the  form  of  the  partitions  between 
the  cells,  which  may  be  plane  or 
curved.  These  partitions  are  call 
ed  floats.  In  this  wheel  the 
water  is  delivered  at  the  bottom, 
and  striking  against  the  floats 
with  a  velocity  due  to  the  height 
Fig.  880.  of  the  water  in  the  reservoir,  A, 

rotary  motion  is  produced.    In  this  wheel,  the  water  acts  solely  by 

its  impulse. 

The  breast  wheel  differs  from  the 

overshot  wheel  in  having  the  water 

delivered  into  the  buckets  at  a  lower 

level,  and  in  being  provided  with  a 

casing,  or  trough,  A,  called  a  breast, 

nearly  fitting  the  periphery  of  the 

wheel  which  revolves  within  it.   In 

this  wheel  the  water  acts  both  by 

its  weight  and  its  impulse.  Fig.  881. 


What  is  a  water-wheel?  How  many  classes?  The  principal  varieties  of 
vertical  wheels?  Describe  the,  overshot  ^cheel,  its  drum,  crowns,  and  buckets.  De 
scribe  the  undershot  ichcel.  The  breast  wheel. 


490 


roruLAu  PHYSICS. 


A  description  of  turbines,  which  are  more  complex  than  vertica] 
wheels,  does  not  fall  within  the  scope  of  the  present  work. 

The  Steam-Engine. 

472.  A  STEAM-ENGINE  is  a  combination  of  pieces  for 
utilizing  the  expansive  force  of  steam  and  converting  it  into 
a  motive  power. 

In  fhe  following  pages  only  a  few  of  the  most  prominent 
points  are  explained.  A  complete  discussion  would  require  more 
space  than  can  be  given  within  the  limits  assigned  to  this  treatise. 
For  further  details  the  pupil  must  consult  those  works  in  which  the 
eubject  of  steam  is  treated  as  a  specialty. 


Steam. 

473.  Let  AB  represent  a  glass  tube  of  uniform  bore,  and  0,  a 
piston,  fitting  it  steam-tight,  and  suppose  a  little  wa 
ter  to  be  in  the  tube  below  the  piston.  If  heat  be 
applied  to  the  bottom  of  the  tube,  by  means  of  a 
spirit-lamp,  the  water  will  be  converted  into  steam, 
and  the  piston  will  be  driven  to  the  top  of  the  tube. 
If  the  lamp  be  removed,  and  the  tube  allowed  to  cool, 
the  steam  will  be  condensed,  and  the  pressure  of  the 
atmosphere  will  drive  the  piston  back  to  its  original 
position.  By  again  applying  heat  and  withdrawing 
it,  the  operation  may  be  repeated,  and  so  on  indef 
initely.  This  simple  experiment  involves  the  funda 
mental  idea  of  the  steam-engine. 

Under  the  ordinary  pressure  of  the  atmosphere,  a 
cubic  inch  of  water  gives  1,700  cubic  inches,  or 
nearly  a  cubic  foot,  of  steam.  In  this  case  the  expansive  force  of 
the  steam  is  in  equilibrium  with  the  pressure  of  the  atmosphere,  and  it 
is  said  to  have  a  tension  of  15  Ibs.  to  the  square  inch.  If  a  cubic  inch 
of  water  be  converted  into  steam,  under  a  pressure  of  two  atmos 
pheres,  it  will  yield  but  850  cubic  inches  of  steam,  -but  the  tension 
will  now  be  30  Ibs.  to  the  inch. 

(473.)  What  is  a  steam-engine?  (4:73.)  Describe  the  experiment  illustrating  the 
idea  of  the  steam-engine.  How  many  cubic  inches  of  steam  does  a  cubic  inch  of 
water  ffire  under  a  pressure  of  15  Us.  to  the  inch?  How  many  under  a  pressure 
of  30  Ibs.  to  the  inch  ? 


MOVERS.  491 

In  general,  tlie  t  olunie  of  steam  yielded  by  a  given  volume  of  water 
varies  inversely  as  the  pressure  under  which  it  is  generated,  and  in 
all  cases  the  tension  of  the  steam  is  equal  to  this  pressure.  Hence, 
the  quantity  of  work  arising  from  the  conversion  of  a  given  bulk  of 
water  into  steam  is  always  the  same,  no  matter  what  may  be  the 
pressure  under  which  it  is  generated.  In  round  numbers,  we  may 
say,  that  the  conversion  of  a  cubic  inch  of  water  into  steam  pro 
duces  a  quantity  of  work  sufficient  to  raise  a  weight  of  one  ton 
through  a  height  of  one  foot. 

It  is  found  that  the  quantity  of  heat  necessary  to  convert  a  cubit) 
inch  of  water  at  32°  F.  into  steam  is  constantly  the  same,  no  mat 
ter  what  may  be  the  pressure.  Hence,  so  far  as  fuel  and  work  are 
concerned,  it  is  of  no  consequence  what  the  pressure  may  be. 

It  was  to  utilize  the  immense  expansive  force  of  steam  that  the 
steam-engine  was  devised. 

Varieties  of  Steam-Engine. 

474.  Steam-engines  may  be  either  condensing  or  non- 
condensing.  In  the  former,  the  steam,  after  having  acted 
upon  the  piston,  is  condensed,  and  the  warm  water  returned 
to  tho  boiler ;  in  the  latter,  the  steam  is  not  condensed, 
but  after  having  acted  upon  the  piston,  is  blown  off  into  the 
air.  In  condensing  engines,  steam  may  be,  and  generally  is, 
used  of  a  lower  tension  than  15  Ibs.  to  the  inch,  in  which 
case  the  engines  are  called  low-pressure  engines.  In  non- 
condensing  engines,  steam  is  always  used  of  a  tension  greater 
than  15  Ibs.  to  the  inch,  and  the  engines  are  then  called  high- 
pressure  engines. 

Condensing  engines  are  Liore  economical  of  fuel,  but  they  are 
heavier  and  more  complex  in  their  construction.  Hence  they  ar^ 
generally  used  as  stationary  engines.  Non-condensing  engines  aix 
used  for  locomotives,  and  where  fuel  is  cheap  they  are  often  emp!'*™"*'1 
as  stationary  engines. 

The  efficiency  of  a  steam-engine  is  measured  in  terms  of  a  unit 

Now  much  work  does  the  evaporation  of  1  cubic  inch  of  water  produce  ?  What  is 
the  rule  for  the  heat  required  under  different  pressure  f  (474:.)  What  is  a  con 
densing  engine  ?  A  non-condensing  engine  ?  High-pressure  engine  ?  Low-pressure 
engine  ?  Advantages  of  each  ?  What  is  a  horsepower  f 


492  POPULAR   PHYSICS. 

called  a  horse-power,  that  is,  a  force  which  is  capable  of  raising  a 
weight  of  33,000  Ibs.  through  a  height  of  one  foot  in  one  minute. 
Thus  an  engine  that  can  perform  a  work  equivalent  to  raising  33,000 
Ibs.  through  10  feet  in  one  minute,  is  said  to  be  an  engine  of  10 
horse-power. 

Boilers  and  their  Appendages. 

475.  The  BOILER  is  a  shell  of  metal,  generally  of  wrought 
iron,  but  sometimes  of  copper,  in  which  steam  is  generated. 

Boilers  are  made  of  various  shapes.  One  of  the  simplest,  has  the 
form  of  a  cylinder,  with  rounded  ends ;  it  is  set  in  a  furnace,  as  shown 
in  Fig.  85.  Sometimes  two  smaller  cylinders,  also  with  rounded 
ends,  called  heaters,  are  placed  below  the  main  shell  and  connected 
with  it  by  suitable  pipes.  The  object  of  this  arrangement  is  to  in 
crease  the  heating  surface.  In  the  Cornish  boiler  the  cylindrical  shell 
has  a  large  flue  passing  through  it,  containing  an  internal  furnace. 
Sometimes  two  such  flues  exist.  The  tubular  boiler  has  a  great 
number  of  tubes,  or  flues,  passing  through  it,  for  transmitting  the 
flame  and  heated  gases  from  the  furnace. 

The  principal  appendages  of  the  boiler  are  the  furnace,  vc  fireplace, 
with  its  flues  and  dampers  for  regulating  the  draft ;  ih&feed  apparatus, 
by  which  water  is  introduced  to  supply  the  place  of  that  converted 
into  steam ;  the  safety  valve,  to  guard  against  danger  of  explosion 
(see  Art.  229)  ;  the  manometer,  for  measuring  the  tension  of  the 
steam  in  the  boiler  (see  Arts.  123  and  124)  ;  the  steam-guage,  to  in 
dicate  the  height  of  the  water  in  the  boiler;  the  T)low-off  apparatus, 
usually  a  cock  near  the  bottom  of  the  boiler,  which,  when  opened, 
permits  the  pressure  of  the  steam  in  the  boiler  to  force  out  the  sedi 
ment  and  impurities  that  collect  there;  and  the  steam-pipe,  to  con 
vey  the  steam  from  the  boiler  to  the  engine  proper. 

The  boiler  and  its  appendages  are  variously  arranged  in  different 
engines,  the  object  in  all  cases  being  to  obtain  the  greatest  amount 
of  steam  with  a  given  quantity  of  fuel.  In  stationary  engines,  the 
furnace  is  usually  made  of  brick  or  some  other  bad  conductor  of 
heat,  and  the  flues  are  so  arranged  as  to  bring  the  flame  and  heated 
gases  in  contact  with  as  large  a  portion  of  the  boiler  as  possible.  To 

(475.)  What  ia  a  boiler?  Describe,  some  of  the  forms  of  boilers.  What  are  th« 
principal  appendages  of  boilers,  and  what  are  their  uses?  What  arrangements 
ar«3  made  for  economizing  heat  in  stationary  engines  T 


PEIME   MOVERS.  493 

pi  event  waste  of  heat,  the  exposed  surface  of  the  boiler  is  covered 
with  a  jacket  of  coarse  felt.  In  locomotive  engines,  the  fire-box  ia 
made  of  boiler-iron,  and  is  so  constructed,  that  it  is  nearly  surround 
ed  by  the  water  in  the  boiler.  An  additional  heating  surface  is 
also  obtained  by  means  of  flues,  running  through  the  boiler,  for  con 
veying  the  flames  and  heated  gases. 

To  guard  against  explosion  from  too  great  a  pressure  of  steam 
within  the  boiler,  the  safety  valve  is  employed.  In  addition  to  this, 
a  fusible  safety  plug  is  sometimes  used.  This  consists  of  a  plug  of 
metal,  inserted  in  the  boiler,  which  is  capable  of  being  fused  at  that 
temperature  beyond  which  there  is  danger  of  explosion.  If  the  tem 
perature  is  raised  above  this  limit,  the  plug  melts  and  falls  out,  per 
mitting  the  water  and  steam  to  escape  through  the  hole  which  it  leaves. 


Mechanism  of  the  Condensing  Engine. 

476.  The  essential  parts  of  a  condensing  engine  are  shown  in 
Fig.  333.  The  figure  is  only  intended  to  illustrate  the  principles  of  the 
engine,  and,  for  the  purpose  of  illustration,  the  parts  are  arranged  in 
such  a  manner  as  best  to  exhibit  them  at  a  single  view. 

The  principal  parts  of  the  condensing  engine  are  the  following  : 

The  cylinder,  shown  on  the  left,  with  a  portion  broken  away. 

The  piston,  P,  which  receives  the  action  of  the  steam,  alternately 
on  its  upper  and  lower  faces,  and  is  thereby  moved  up  and  down  in 
the  cylinder. 

The  steam-chest,  5,  into  which  the  steam  from  the  boiler  enters 
through  the  steam-pipe  at  o,  and  from  which  it  passes  through  the 
steam  passages,  alternately  to  the  upper  and  lower  ends  of  the 
cylinder. 

The  sliding  valve,  moved  up  and  down  by  the  rod,  m,  which 
alternately  opens  a  communication  between  the  steam-chest  and  the 
two  steam  passages  leading  to  the  top  and  bottom  of  the  cylinder. 

The  eduction  pipe,  U,  connecting  with  the  cylinder  at  a,  by  which 
the  steam,  after  having  acted  upon  the  piston,  is  conducted  into  the 
condenser,  0. 

The  piston-rod,  A,  working  through  a  pacTcing-lox,  d,  which  trans 
mits  the  motion  of  the  piston  to  the  working-beam,  L. 

The  parallel  lars,  DD,  and  the  radial  fozrs,  C,E,  which  keep  the 


In  locomotive  engines  f    Describe  the  safety  plug.     (4V6.)  Describe  the  principal 
parts  of  a  condensing  engine  and  their  uses. 


POPULAR   PHYSICS. 

piston-rod  from  pressing  against  the  side  of  the  packing-box, 
arrangement  is  called  Watt's  parallel  motion. 

The  connecting  rod,  I,  which  transmits  the  motion  of  the  working- 
beam  to  the  crank  arm,  K,  and  through  it  imparts  a  motion  of 
rotation  to  the  shaft  of  the  engine. 


Fig.  868. 

The  fly-wheel,  V,  which  obviates  to  a  certain  extent  the 
irregularities  of  motion  in  the  engine. 

The  excentric,  e,  which,  acting  like  a  crank,  produces  a  backward 
nnd  forward  motion  in  the  connecting  rod,  Z.  This  rod  acting  on 

Describe  the  principal  parts  of  a  condensing  engine  and  their  uses. 


PRIME   MOVERS.  495 

the  "bent  lever,  Y,  causes  the  rod  m,  of  the  sliding  valve,  to  move  up 
and  down. 

The  cold-water  pump,  K,  worked  by  the  rod,  n,  which  draws  cold 
water  from  a  reservoir,  and  forces  it  through  the  pipe,  T,  into  the 
condenser.  This  pipe,  terminating  within  the  condenser  in  a  rose, 
delivers  the  water  in  the  form  of  a  shower,  and  condenses  the 
steam. 

The  air-pump,  M,  worked  by  the  rod,  F,  which  draws  the  hot 
water  and  the  air  that  is  mixed  with  it  from  the  condenser,  and  forces 
it  into  the  hot  well,  N". 

The  feed-pump,  Q,  worked  by  the  rod,  G,  which  draws  the  water 
from  the  hot  well  and  forces  it  into  the  boiler. 

To  explain  the  action  of  the  engine,  let  the  position  of  the  parts 
be  as  represented  in  the  figure.  The  steam  entering  the  steam- 
chest  finds  the  upper  passage  open,  and  flowing  through  it,  acts  upon 
the  upper  face  of  the  piston  and  drives  it  to  the  bottom  of  the  cylin 
der.  The  steam  below  the  piston  meanwhile  flows  through  the  lower 
passage,  and  entering  the  eduction  pipe  at  a,  is  conveyed  to  the  con 
denser,  where  it  is  condensed.  "When  the  piston  reaches  the  bottom 
of  the  cylinder,  the  excentric  acts  upon  the  bent  lever  to  open  the 
lower  and  close  the  upper  passage.  The  steam  from  the  steam-chest 
now  flows  through  the  lower  passage,  and  acting  upon  the  lower 
face  of  the  piston,  forces  it  to  the  top  of  the  cylinder.  Meantime  the 
steam  above  the  piston,  flowing  down  the  upper  passage,  enters  the 
eduction  pipe  and  is  conveyed  to  the  condenser.  When  the  piston 
reaches  the  top  of  the  cylinder,  the  excentric  again  acts  to  change 
the  position  of  the  sliding  valve,  and  thus  the  motion  of  the  piston 
is  continued  indefinitely. 


The  Locomotive. 

477.  The  figure  represents  a  section  of  a  locomotive,  the 
principal  parts  of  which  are  the  following : 

The  loiler,  BB,  with  its  flues,  pp,  and  safety-valve,  M.  The  dotted 
line  represents  the  height  of  the  water  in  the  boiler. 

The  fire-box,  A,  communicating  with  the  smoTce-lox,  0,  by  means 
of  the  fiues,  pp.  The  fire-box  has  a  double  wall,  the  interval  being 


fteplain  the  action  of  a  condensing  engine.    (477.)  Explain  the  principal  part* 
of  a,  locomotive  engine. 


496 


POPULAK   PHYSICS. 


TKIME   MOVERS. 


497 


filled  with  water  and  communicating  with  the  boiler.  E  is  tho 
grate,  and  D  the  door  for  the  supply  of  fuel. 

The  steam-pipe,  SS,  conveys  the  steam  from  the  steam-dome  to 
the  steam-chest.  It  may  be  closed  by  a  valve,  V,  worked  by  a 
lever,  L. 

The  steam  -dome  is  an  elevated  portion  of  the  boiler,  the  object  of 
which  is  to  permit  the  steam  to  enter  the  steam-pipe  without  any 
admixture  of  water,  as  might  be  the  case  were  the  steam  taken  from 
a  lower  level. 

The  cylinder,  the  piston,  P,  and  the  piston-rod,  R,  are  similar  to 
the  corresponding  parts  of  the  condensing  engine. 

The  Mast  pipe,  L,  through  which  the  steam  is  blown  off  after 
having  acted  upon  the  piston,  terminates  in  the  smoke-box,  and  the 
blast  of  steam  from  it  serves  to  increase  the  draft  of  air  through  the 
flues,  and  thus  promotes  the  combustion  of  fuel. 

The  connecting  rod,  G,  transmits  the  motion  of  the  piston  to  the 
crank  arm,  by  means  of  which  a  rotary  motion  is  imparted  to  the 
driving  wheels  of  the  locomotive. 

The  manner  in  wrhich  steam  acts  to  impart  motion  to  the  piston  is 
the  same  as  in  the  engine  already  described. 


The  Hydraulic  Ram. 

478.  The  HYDRAULIC  RAM  is  a  machine  for  raising  water  by  means 
of  shocks,  caused  by  sudden  stoppages  of  a  stream  of  water.  It  consists 
of  a  reservoir,  B,  with  a  supply  pipe,  A,  and  an  orifice,  D,  which 
may  be  closed  by  a  spherical  valve.  Attached  to  the  reservoir  is  an 
air-vessel,  G,  with  a  valve,  E,  and  a 
delivery  pipe,  II. 

The  stream  of  water  entering  the 
reservoir  through  the  supply  pipe, 
forces  the  valve  D  into  its  place,  and 
closes  the  orifice.  The  sudden  stop 
page  of  the  water  causes  a  shock  which 
forces  a  portion  of  water  into  the  air- 
vessel  through  the  opening,  E.  The 
equilibrium  being  restored,  the  valve  D  FiS- 835- 

falls,  as  does  the  valve  E,  and  immediately  a  second  shock  ensues, 


Explain  the  action  of  a  locomotive  engine. 
Explain  its  construction  and  use. 


(478.)  What  is  the  hydraulic  ram  I 


4:98  POPULAK  PHYSICS. 

and  a  second  supply  of  water  is  forced  into  the  air-vessel,  and  so  on 
indefinitely.  The  water  forced  into  the  air-vessel  compresses  the 
air  in  the  upper  portion  of  it,  until  its  elastic  force  becomes  sufficient 
to  force  a  jet  of  water  up  the  delivery  pipe.  The  delivery  once 
commenced  will  continue  as  long  as  the  machine  remains  in  order. 
Only  a  small  portion  of  the  water  which  enters  the  reservoir  is  raised 
into  the  delivery  pipe. 


1  I  D  E  X. 


CHAPTER     I. 


and  General  Proper 
ties  of  Matter. 
Definition    of    Physics.  —  Physical 

Agents 11 

Definition  of  a  Body 

Mass  and  Density 12 

Classification  of  Bodies 12 

General  Properties  of  Bodies 13 

Magnitude  and  Form 13 

Impenetrability 13 

Inertia 14 

Porosity 15 

Divisibility 17 

Compressibility 18 

Disability 19 

Elasticity 20 


Mechanical  Principles. 

Definition  of  Mechanics 22 

Rest  and  Motion 22 

Different  kinds  of  Motion 20 

Uniform  Motion.— Velocity 23 

Varied  Motion.  —  Accelerated  and 

Retarded  Motion 23 

Forces,  Powers,  and  Resistances 24 

Distinctive  Characteristics  of  Forces  25 

Resultant  and  Component  Forces. . .  2(5 

Parallelogram  of  Forces 27 

Practical  Example  of  Composition  of 

Forces 28 

Practical  Example  of  Resolution  of 

Forces 29 

Resultant  of  Parallel  Forces 29 

Equilibrium  of  Forces 30 

Centrifugal  and  Centripetal  Forces.  30 
Some  Effects  of  the  Centrifugal 

Force 32 

Machines 33 

The  Lever 34 

Conditions  of  Equilibrium  of  the 

Lever 35 

Examples  of  Levers 37 

Other  Machines 38 


PAGE 

Principles  Dependent  on  the  At 
traction  of  Gravitation. 

Universal  Gravitation 39 

Effect  of  Gravitation  on  the  Planets  40 

Force  of  Gravity 40 

Vertical  and  Horizontal  Lines 41 

The  Plumb-Line 42 

Weight 42 

Centre  of  Gravity 43 

Equilibrium  of  heavy  Bodies 44 

Different  kinds  of  Equilibrium 46 

Stability  of  Bodies 48 

The  Balance 50 

Requisites  for  a  good  Balance 52 

Methods  of  Testing  a  Balance 53 

M  ethod  of  weighing  correctly  with  a 

false  Balance 53 

Laws  of  falling  Bodies 53 

The  Inclined  Plane 55 

Verification  of  the  third  Law  of  fall 
ing  Bodies 56 


Applications  of  the  Inclined  Plane . . 

The  Pendulum 

Simple  and  Compound  Pendulums.. 
Laws  of  Oscillation  of  the  Pendulum 

Applications  of  the  Pendulum 

The  Metronome 


Principles  Dependent  on  Molecular 
Action. 

Molecular  Forces 64 

Cohesion 65 

Adhesion 65 

Capillary  Forces 66 

Applications  of  Capillarity 67 

Absorption 63 

Imbibition 63 

Properties  of  Solids  Dependent  on 
Molecular  Action. 

Tenacity. 70 

Hardness 71 

Ductility 71 

Malleability 72 


500 


INDEX. 


CHAPTBB     II. 


General  Principles. 

Definition  of  Hydrostatics  and  Hy 
drodynamics 73 

Properties  of  Liquids 73 

Transmission  of  Pressures. — Princi 
ple  of  Pascal 74 

Pressure  due  to  the  Weight  of  Li 
quids 75 

Lateral  Pressures. — Reaction  Wheel  76 

Pressure  Upwards 77 

Pressure  on  the  Bottom  of  a  Vessel 

independent  of  its  Shape 73 

Pascal's  Experiment. 79 

Hydraulic  Press 79 


Equilibrium  of  Liquids. 

Conditions  of  Equilibrium 82 

Level  Surface 82 

Equilibrium  of  Liquids  in  Communi 
cating  Vessels 83 

Case  of  Vessels  containing  Liquids 

of  different  Densities 84 

Equilibrium  of  Heterogeneous  Li 
quids 85 


PAGE 

Applications  of  tl>A  Principle  of 
Equilibrium. 

The  Water  Level 80 

The  Spirit  Level 87 

Springs  — Fountains. — Rivers 88 

Artesian  Wells 89 

Pressure  on  Submerged  Bodies. 

Principle  of  Archimedes 00 


Hydrostatic  Balance 

Cylinder  and  Bucket  Experiment.. 
Floating  Bodies.— Principles  of  Flo 
tation 

Illustration  of  the  Principles  of  Flo 
tation 


98 

94 

Swimming  Bladder  of  Fishes 95 

Swimming 95 

Specific  Gravity  of  Bodies. 

Definition  of  Specific  Gravity. 96 

Specific  Gravity  of  Solids 97 

Specific  Gravity  of  Liquids 99 

Beaume's  Areometer 102 

The  Alcoholometer 1 03 

The  Lactometer. 104 


CHAPTER     III. 


The  Atmosphere. 

General  Properties  of  Gases  and  Va 
pors 105 

Description  of  the  Atmosphere 106 

Expansive  Force  of  Air 106 

Weight  of  Air 107 

Composition  of  the  Atmosphere. . . .  108 

Atmospheric  Pressure 108 

Bursting  a  Membrane 109 

The  Magdebourg  Hemispheres 110 

Torricellian  Tube.— Measure  of  the 

Atmospheric  Pressure Ill 

Pascal's  Experiments 113 

The  Barometer 113 

The  Cistern  Barometer 114 

The  Siphon  Barometer 116 

Properties  of  a  good  Barometer. 116 

Mean  Height  of  the  Barometer 117 

Causes  of  Barometrical  Fluctuations  118 

The  Index  Barometer 118 

Measure  of  Mountain  Heights  by  the 

Barometer 120 

Height  of  the  Atmosphere 121 

Atmospheric  Pressure  transmitted  in 

all  Directions 121 

Pressure  on  the  Human  Body 122 

Measure  of  the  Elastic  Force  of  Gases. 

Mariotte's  Law 1 24 

Marietta's  Tube  .124 


Manometers 126 

The  Open  Manometer 126 

The  Closed  Manometer 127 

Application  to  Pumps  and  other 

Machines. 

The  Air-pump .»  128 

Measure    of    the    Rarefaction   pro 
duced 132 

Experiments  with  the  Air-pump 132 

Preservation  of  Food  in  a  Vacuum.  133 

The  Condenser. 134 

Artificial  Fountains 135 

Hero's  Fountain 135 

Intermittent  Fountain 137 

The  Atmospheric  Inkstand 138 

Water  Pumps 139 

The  Sucking  and  Lifting  Pump 139 

The  Forcing  Pump 142 

The  Fire  Engine 144 

The  Sucking  and  Forcing  Pump. ...  146 

The  Siphon 147 

Application  to  Ballooning. 

Buoyant  Effort  of  the  Atmosphere. .  149 

The  Balloon 150 

Manner  of  filling  a  Balloon  and  mak 
ing  an  Ascent 151 

The  Parachute 153 

Remarkable  Balloon  Ascension 155 


INDEX. 


501 


CHAPTER      IV. 


Production  and  Propagation  of 
Sound. 

Definition  of  Acoustics 156 

Definition  of  Sound 156 

Propagation  in  Air.—Sonml- Waves  157 

Superposition  of  Sound-Waves 158 

Sound  is  not  propagated  in  a  Vacu- 

um 159 

Propagation  of  Sound  in  Liquids  and 

Solids 159 

Velocity  of  Sound  in  the  Air 160 

Velocity  of  Sound  in  Liquids  and 

Solids 162 

Eeflection  of  Sound 162 

Echoes 162 

Eesonance 163 

Intensity  of  Sound 164 

Causes  that  Modify  the  Intensity  of 

Sound 164 

Intensity  of  Sounds  in  Tubes 165 


PAGE 

The  Speaking  Trumpet 1 66 

The  Ear  Trumpet 16S 

Musical  Sounds. 
Difference  between  a  Musical  Sound 

and  a  Noise 163 

Pitch  of  Sounds.— Music 169 

Limits  of  perceptible  Sounds 169 

Musical  Scale.— Gamut 170 

Intervals.— Accords 171 

The  Tuning  Fork 172 

Transverse  Vibrations  of  Cords 173 

Laws  of  Transversal  Vibrations  of 

Cords 173 

Verification  of  the  Laws  of  Vibration  174 

Stringed  Instruments 175 

Sound  from  Pipes 176 

Pipes  with  fixed  Mouth-pieces 176 

Eced  Pipes 178 

The  Bellows 181 

Wind  Instruments...                        .  181 


CHAPTER     V. 


General  Properties  of  Heat. 

Definition  of  Heat 182 

Theories  of  Heat 182 

General  Effects  of  Heat 1S3 

Expansion  of  Bodies  by  Heat 183 

Sensible  and  Latent  Heat. — Temper 
ature  187 

Thermometers. 

The  Thermometer 187 

Method  of  making  a  Thermometer. .  1 88 

Method  of  Graduation 188 

Thermometer  Scales 189 

Conversion  of  Centigrade  and  Eeau- 

rnur's  Degrees  into  Fahrenheit's. .  191 

Alcohol  Thermometers 192 

Eelative  advantages  of  Mercurial  and 

Alcohol  Thermometers 192 

Eules  for  using  a  Thermometer 193 

The  Differential  Thermometers 1 98 

Eumford'sDifferentinl  Thermometer  194 

Leslie's  Differential  Thermometer. .  195 

Pyrometer 195 

Radiation  of  Heat. 

Propasation  of  Heat 196 

Laws  of  Eadiant  Heat 196 

Mutual  Exchange  of  Heat  between 

bodies 197 

Reflection,  Absorption,  Emission,  and 
Conductilility. 

Eeflection  of  Heat 198 

Laws  which  govern  the  Eeflection  of 

Heat 199 


Eeflection  of  Heat  from   Concave 

Mirrors 200 

Eeflecting  Power  of  Different  Sub 
stances  202 

Power  of  Absorption 203 

Radiating  Power 204 

Modifications     of    the    Reflecting 

Powers  of  Bodies 205 

Applications  of  the  preceding  Prin 
ciples    206 

Conductivity  of  Solid  Bodies 207 

Conductivity  of  Liquids.— Convec 
tion  208 

Conductivity  of  Gases 209 

Applications  of  the  preceding  Prin 
ciples 209 

Laws  of  Expansion  of  Solids,  Liquids, 
and  Gases. 

Laws  of  Expansion  of  Solids 210 

Applications 212 

Compensating  Pendulum 213 

Laws  of  Expansion  of  Liquids 214 

Maximum  Density  of  Water 215 

Law  of  Expansion  of  Gases 216 

Applications 216 

Density  of  Gases 218 

Change  of  State  of  Bodies  ly  the  Action 
of  Heat. 

Fusion 219 

Latent  Heat  of  Fusion 220 

Congelation.— Solidification 221 

Crystallization 222 

Freezing  Mixtures 223 


502 


INDEX. 


PAGE 

Vaporization.  —  Elastic  Force   of 

Vapors. 
Vaporization.  —  Volatile    and  Fixed 

Liquids 223 

Evaporation  under  Pressure 224 

Instantaneous     Evaporation    in     a 

Vacuum 226 

Limit  of  the  Tension  of  Vapors 227 

Saturation 227 

Causes  that  Accelerate  Evaporation.  228 

Ebullition 229 

Causes  that  Modify  the  Boiling  Point 

of  Liquids 231 

Papin's  Digester 234 

Measure  of  the  Elastic  Force  of  Vapor  235 

Latent  Heat  of  Vapors 236 

Examples  of  Cold  produced  by  Heat 

becoming  Latent 237 

Congelation  of  Water  and  Mercury 

in  a  Vacuum 237 

Condensation  of  Gases  and   Vapors. — 
Specific  Heat. 

Causes  of  Condensation 233 

Heat  developed  by  Condensation.. ..  240 

Heating  by  Steam 240 


PAGE 

Distillation 240 

The  Alembic 241 

Liquefaction  of  Gases 242 

Specific  Heat  of  Solids  and  Liquids.  243 

Hygrom  etry.— Eain.— De  u\—  Win  ds. 

Hygrometry 245 

Moisture  in  the  Air,  and  its  Effects..  245 

The  Hygroscope 240 

The  Hair  Hygrometer 247 

Hygrometric  state   of  the   Atmos 
phere 248 

Formation  of  Fogs  and  Clouds 248 

Kain 250 

Dew  and  Frost 251 

Snow  and  Hail 251 

Winds 252 

Causes  of  Winds 252 

Regular,     Periodic,     and     Variable 

Winds 253 

The  Simoon.— The  Sirocco 254 

Velocity  of  Winds 254 

Sources  of  Heat  and  Cold. 

Sources  of  Heat 255 

Sources  of  Cold 257 


CHAPTER      VI. 


General  Principles. 

Definition  of  Optics 259 

Definition  of  Light 259 

Two  Theories  of  Light 259 

Sources  of  Light 260 

Media.  —  Opaque    and    Transparent 

Bodies 261 

Absorption  of  Light 262 

Rays  of  Light.— Pencils.— Beams. . .  262 

Velocity  of  Light 263 

Intensity  of  Light.— Photometry. ...  265 

Reflection  of  Light.— Mirrors. 

Reflection  of  Light 267 

Definitions  of  Terms 268 

Laws  of  Reflection 269 

Direction  in  which  Objects  are  seen.  269 

Mirrors 270 

Plane  Mirrors 270 

Images  Formed  by  Plane  Reflectors  271 

Nature  of  the  Images  formed 273 

Multiple     Images     from     Looking- 
glasses  273 

Reflection  by  Transparent°Bodies! .' .'  274 

Curved  Mirrors 274 

Concave  Mirrors '..... .......  275 

Principal  Focus  of  a  Concave  Mirror  276 

Conjugate  Foci 277 

Formation  of  Images  by  Concave  Re 
flectors 279 

Real  images  ....'.'.'..'..'..'.'. 279 

Virtual  Images 281 

Formation  of  Images  by  Convex  Re 
flectors  . . ,  .282 


Refraction  of  Light.— Lenses. 

Refraction 283 

Definitions 283 

Laws  of  Refraction 284 

Refractive  Power  of  Bodies 285 

Experimental  proofs  of  Refraction . .  28li 

Some  effects  of  Refraction 2S7 

Total  Reflection 289 

Mirage 290 

Media  with  Parallel  Faces 292 

Prisms 292 

Course  of  Luminous  Rays  in  a  Prism  293 

Lenses 294 

Classification  of  Lenses 295 

Definitions  of  Terms 297 

Action  of  Convex  Lenses  on  Light. .  298 

Principal  Focus 298 

Conjugate  Foci 299 

Formation    of  Images    by  Convex 

Lenses 301 

Formation    of  Images  by  Concave 

Lenses 305 

Burning-glasses 306 

Light-houses 807 

Decomposition  of  Light. — Colors  of 
Bodies. 

Solar  Spectrum 310 

Fraunhofer's  lines.— Spectroscope.  312 

Spectrum  Analysis 314 

Recomposition  of  Light 314 

Color  of  Opaque  Bodies 31(5 

Colors  of  Transparent  Bodies 318 

Complementary  Colors 31S 


503 


PAGE 

Accidental  Images. 318 

Accidental  Fringes 318 

The  Rainbow 320 

Chromatic  Aberration 321 

Achromatic  Combinations 322 

TJieory  and  Construction  of  Optical 
Instruments. 


Optical  Instruments 

Telescopes 

The  Galilean  Telescope 

The  Astronomical  Telescope 

The  Terrestrial  Telescope 

Kefl ecting  Telescopes 

Newtonian  Telescope 

Herschel's  Telescope 

Lord  Ross's  Telescope 

Microscopes 


823 
823 
824 
825 

327 

328 
328 


881 


PAGE 

The  Simple  Microscope 331 

The  Compound  Microscope 333 

The  Magic  Lantern 334 

The  Phantasmagoria 336 

The  Polyrama  and  Dissolving  Views  333 

The  Photo-Electric  Microscope 338 

The  Diorama 340 

The  Camera  Obscura 342 

Manner  of  rendering  the  Image  erect  846 

The  Portable  Camera  for  Artists.. ..  34T 

The  Daguerreotype 347 

Process  of  Daguerre 348 

Photography 850 

Structure  of  the  Eye 354 

The  Mechanism  of  Vision 354 

Limit  of  Distinct  Vision.— Defects  of 

Sight 354 

Vision  Avith  two  Eyes .354 

The  Stereoscope 356 


CHAPTER     VII 


General  Properties  of  Magnets. 

Definition  of  Magnetism  ...........  .  358 

Magnets  ...........................  353 

Distribution  of  Force  in  Magnets.  .  .  359 

Hypothesis  of  two  Magnetic  Fluids.  861 

Laws  of  Attraction  and  Repulsion.  .  .  863 
Magnetic  and  Magnetized  Bodies  ---- 

The  Coercive  Force  ................ 


363 


Terrestrial  Magnetism.— Compasses. 
Directive  Force  of  Magnets 366 


Magnetic  Meridian. — Declination. — 

Variations 367 

The  Compass 369 

The  Dipping  Needle 370 

4 

Methods  of  Imparting  Magnetism,. 

Magnetizing  by  Terrestrial  Induc 
tion 371 

Magnetizing  by  Friction 372 

Bundles  of  Magnets.— Armatures. . .  374 


CHAPTER     VIII. 


Fundamental  Principles. 

Definition  of  Electricity 376 

Discovery  of  Electrical  Properties . .  376 

Sources  of  Electricity 377 

Electroscope.— Electrical  Pendulum  378 

Two  kinds  of  Electricity 378 

Hypothesis  of  two  Electrical  Fluids.  380 
Laws  of  Electrical   Attraction  and 

Repulsion 881 

Conductors. — Insulators 381 

Methods  of  Electrifying  Bodies 382 

Accumulation  of  Electricity  on  the 

Surface  of  Bodies 383 

Influence  of  the  Forms  of  Bodies. — 

Power  of  Points 384 

Principle  of  Induction. — Electrical 
Machines. 

Induction 386 

The  Electrical  Machine 388 

Use  of  the  Electrical  Machine 890 

Measure  of  the  Quantity  of  Electri 
city  in  the  Machine 891 

Precautions  in  using  the  Machine ...  89 1 


Electrophoms 393 

Gold-leaf  Electrometer 895 

Method  of  using  the  Gold-leaf  Elec 
trometer 896 

Electrical  Recreations. 

Electrical  Spark.— Electrical  Shock .  397 

The  Electrical  Stool 899 

The  Electrical  Chime 899 

The  Electrical  Puppet 401 

The  Electrical  Wheel 401 

The  Electrical  Egg 402 

The  Electrical  Square 403 

The  Electrical  Cannon 404 

Accumulation  of  Electricity. 

Electrical  Condenser 4^6 

Condenser  of  Epinus  406 

Method  of  using  the  Condenser 407 

Slow  discharge^of  the  Condenser.— 
Instantaneous  discharge. — Dischar 
ger 409 

Limit  of  the  Charge  in  a  Condenser.  410 

The  Lcyden  Jar 416 


504 


INDEX. 


PAGE 

The  Electrical  Battery 412 

Condensing  Electrometer 413 

Effects  of  Accumulated  Electricity. 
Physiological  Effects  of  Electricity. .     415 

Heating  Power  of  Electricity 41T 

Mechanical  Effects  of  Electricity 420 

Atmospheric  Electricity. 
Identity  of  Lightning  and  the  Elec 
trical  Spark 421 


PA92 

Atmospheric  Electricity 422 

Lightning  and  Thunder 423 

The  Thunderbolt- 424 

Effects  of  the  Thunderbolt 424 

Means  of  Safety 425 

The  Return  Shock 426 

Lightning-rods 427 

Electrical  Meteors 4-28 

Hail 4<>3 

The  Tornado 429 

The  Aurora  Boreal  is.. .                     .  430 


CHAPTER     IX 


Fundamental  Principles. 

Galvani's  Experiment 432 

Volta'a  Theory  of  Contact 4*3 

The  Vollaic  Pile 434 

Electrical   Tension    in   the    Pile.— 

Poles.— Electrodes 435 

Electrical  Currents 436 

Chemical  Theory  of  the  Pile 4^6 

The  Carbon  Pile 437 

Applications  of  Galvanic  Electricity. 

Effects  of  the  Galvanic  Battery 433 


Physiological  Effects 439 

Heating  Effects 440 

Illuminating  Effects 441 

Chemical  Effects 442 

Decomposition  of  Water 443 

Decomposition     of     Oxydes      and 

Salts 444 

Application    of  Electricity  to    Gal- 

vanoplasty 445 

Method  of  Electrotyping 445 

Electro-gilding     aiid     Electro-plat 
ing 448 


CHAPTER     X. 


•  Fundamental  Principles. 

Relation  between   Magnetism    and 

Electricity 451 

Action  of  an  Electrical  Current  upon 

aMagnet 452 

Ampere's  Law 453 

Action  of  Magnets  upon  Currents, 
and  of  Currents  upon  Currents. . .     453 

Ampere's  Theory  of  Magnetism 455 

Galvanometer 456 

Galvanic  Multiplier 456 

Uses  of  the  Galvanic  Multiplier.   ...     457 
Magnetizing  by  means  of  an  Electri 
cal  Current, 453 


Electro-Magnetic  Telegraphs.—  TJie 
Electro-Motor. 

The  Electro-Masrnet 459 

The  Electrical  Telegraph 460 

Morse's  Registering  Telegraph 462 

Morse's  Manipulator  and  Receiver..  464 

Velocity.— Submarine  Cables 467 

Electro-Magnetic  Motor 467 

Induction. — Application  to  Medicine. 

Induction  by  Currents 469 

Properties  of  Induced  Currents 470 

Effects  of  Electrical  Currents 472 

Electrical  Fishes 478 


CHAPTER     XI. 


General  Principles. 

Definition  of  a  Machine 475 

Motors 475 

Object  and  Utility  of  Machines 475 

Quantity  of  Work  of  a  Force 476 

Equilibrium  of  a  Machine 476 

Elementary  Machines. 

Mechanical  Powers 477 

The  Cord 477 

The/Lever—Compound  Levers 477 

The  Inclined  Plane 479 

The  Pulley— Single  Fixed  Pulley- 
Single  Movable  Pulley — Combina 
tions  of  Pulleys 479 

The  Wheel  and  Axle— The  Windlass 
and  Capstan  — The  Differential 

Windlass 480 

The  Screw 4^2 

The  Wedge 483 


Hurtful  Resistances. 

Friction 483 

Stiffness  of  Cords 434 

Atmospheric  Resistance 484 

Wheehcork. 

Trains  of  Wheels 485 

Mode  of  Connection 485 

Regulators. 

The  Governor 487 

The  Fly  Wheel 483 

Prime  Movers. 

Definition  of  a  Prime  Mover 488 

Water-Wheels 4S9 

The  Steam-Engine 49:» 

Steam 490 

Varieties  of  Steam-Engine 491 

Mechanism  of  the  Condensing  Engine  493 

The  Locomoiive 495 

The  Hydraulic  Bam 4»T 


\o: 


,  all  IHannersi,  nnrt  all 


HAHONAL       TTTQT'n'D  V      STAIDAED 
SERIES.        JllDlUXlI;    TEiT-BOOKS. 


"History  is  (Philosophy  teaching  by  Examples!' 


THE  UNITED  STATES. 


•• 


MONTEITH,  author  of  the  National  Geographical  Series.  An  elementary  work 
upon  the  catechetical  plan,  with  Maps,  Engravings,  Memoriter  Tables,  etc.  For 
the  youngest  pupils. 

2,  Wlllard's    School    History,   for  Grammar  Schools  and  Academic  classes. 

Designed  to  cultivate  the  memory,  the  intellect,  and  the  taste,  and  to  BOW  the 
seeds  of  virtue,  by  contemplation  of  the  actions  of  the  good  and  great. 

3,  Wlllard's    Unabridged     History,    for  higher  classes  pursuing  a  complete 

course.  Notable  for  its  clear  arrangement  and  devices  addressed  to  the  eye,  with 
a  series  of  Progressive  Maps. 

4,  Summary  of  American  History.    A  skeleton  of  events,  -with  all  the  prom 

inent  facts  and  dates,  in  fifty-three  pages.  May  be  committed  to  memory  ver- 
latim,  used  in  review  of  larger  volumes,  or  lor  reference  bimply.  "  A  miniature 
of  American  History." 

FNfil  AND      '•  Berard's  School   History  of  England,  combining 

UllULnllU"  an  interesting  history  of  the  social   life  of  the  English 

people,  with  that  of  the  civil  and  military  transactions  of  the  realm.  Religion, 
literature,  science,  art,  and  commerce  are  included. 

2    Summary  of  English  and  cf  French   History, 

A  series  of  brief  statements,  presenting  more  points  of 

attachment  for  the  pupil's  interest  and  memory  than  a  chronological  table. 

well-proportional  outline  and  index  to  more  extended  reading. 


ROME 


R'lCOrd's  History  of  Rome.  A  story-like  epitome  of  this  inter 
esting  and  chivalrous  history,  profusely  illustrated,  with  the  legends 
and  doubtful  portions  so  introduced  as  not  to  deceive,  while  adding  extended 
charm  to  the  subject. 

RFNFRAI         Wi!  lard's  Universal   History.    A  vast  subject  eo  arranged 
Ul«llL.IIHl«»  aD(i  illustrated  as  to  be  less  difficult  to  acquire  or  retain.  Its 

•whole  substance,  in  fact,  is  summarized  on  one  page,  in  a  grand  "  Temple  of 

Time,  or  Picture  of  Nations. 

2  General  Summary  of  History.  Ecin^  the  Summaries  of  American,  and 
of  English  and  French  History,  bound  ia  one  volume.  The  leading  events  in 
the  histories  of  these  three  nations  epitomized  in  the  briefest  manner. 


A.   S.   BARNES   &  CO., 


"A  Well  of  Englis 


LITERATURE  AND  BELLES  LETTRES. 


PROFESSOR  CLEVELAND'S  WORKS. 

JL  WHOLE  LIBRARY  IN  FOUR  VOLUMES. 


OF  ENGLISH 
OF  19th  CENT'Y 
OF  AMERICAN 
OF  CLASSICAL 


One  Hundred  and  Twenty  Thousand  of  these  Volumes  have  been  sold, 

and  they  are  the  acknowledged  Standard  wherever 

this  refining  study  is  pursued. 

'  ~PEOF.  JAMES  E.  BOYD'S  WORKS. 

EM3KACIXO 

COMPOSITION,  LOGIC,   LITERATURE,  RHETORIC,   CRITICISM, 
BIOGRAPHY,-— POETRY,  AND  PROSE. 


BOYD'S  COMPOSITION  AND  RHETORIC. 

Remarkable  for  the  space  and  attention  given  to  grammatical  principles,  to  afford  a 
substantial  groundwork ;  also  for  the  admirable  treatment  of  synonyms,  figurative 
language,  and  the  source*  of  argument  and  illustration,  with  notable  exercises  for  pre 
paring  the  way  to  poetic  composition. 

BOYD'S  ELEMENTS  OF  LOGIC. 

explains,  first,  the  conditions  and  processes  by  which  the  mind  receives  ideas,  and 
then  unfolds  the  art  of  reasoning,  with  clear  directions  for  the  establishment  and  con 
firmation  of  sound  judgment.  A  thoroughly  practical  treatise,  being  a  systematic  and 
philosophical  condensation  of  all  that  is  known  of  the  subject. 

BOYD'S  KAMES'  CRITICISM. 

This  standard  work,  as  is  well  known,  treats  of  the  faculty  of  perception,  and  the 
result  of  its  exercise  upon  the  tastes  and  emotions.  It  may  therefore  be  termed  a  Com 
pendium  of  Aesthetics  and  Natural  Morals ;  and  its  use  in  refining  the  mind  and  heart 
e  it  a  standard  text-book. 

BOYD'S  ANNOTATED  ENGLISH  CLASSICS. 


Milton's  Paradise  Lost. 
Young's  Night.  ThougJtts. 
Coii-per's  Task,  Table  Talk,  &c. 


TJiomson's  Seasons. 
PolloU's  Course  of  Time. 
Lord  Bacon's  Essays. 


In  six  cheap  volumes.  The  service  done  to  literature,  by  Prof.  Boyd's  Annotations 
upon  these  standard  writers,  can  with  difficulty  be  estimated.  Line  by  line  their  ex 
pressions  and  ideas  are  analyzed  and  discussed,  until  the  best  comprehension  of  the 
powerful  use  of  language  is  obtained  by  the  learner. 


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